Merge branch 'json-view/22-view-dump-fast' into json-view/15-view-bench

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json_document/erase.md

@@ -86,8 +86,8 @@ view of a *different* document (overloads 1-2 only; overload 3 always starts fro
 
 !!! info "Duplicate keys"
 
-    Overload 1. removes *every* member with `key`, not just the last one that lookups find -- unlike [`set`](set.md),
-    which assigns that member and drops the rest. This is why it returns a count rather than a single view: there may be
+    Overload 1. removes *every* member with `key`, not just the first -- unlike [`set`](set.md), which assigns the
+    first occurrence and drops the rest. This is why it returns a count rather than a single view: there may be
     more than one member removed, or none.
 
 Like [`set`](set.md) and [`push_back`](push_back.md), `erase` never moves an element's *value*: a view still

docs/mkdocs/docs/api/basic_json_document/set.md

@@ -23,8 +23,7 @@ has `set`; calling it on a read-only `basic_json_document` fails to compile (`#!
 
 1. Replaces the value `target` refers to with `value`.
 2. Sets the member `key` of the object `object` to `value`: assigns it if `object` already has a member with this
-   key -- the last one, should the key occur more than once (the member
-   [`operator[]`](../basic_json_view/operator%5B%5D.md) returns), and the other duplicates are then dropped (see the
+   key -- the first one, should the key occur more than once, and the later duplicates are then dropped (see the
    [Notes](#notes) below) -- or appends a new member at the end otherwise. A [null](../basic_json_view/is_null.md)
    `object` first becomes an empty object.
 3. Assigns `value` to the element at index `idx` of the array `array`, which must already exist (`#!cpp idx <
@@ -147,13 +146,12 @@ document") if `target`/`object`/`array` is a [discarded](../basic_json_view/is_d
 
 !!! info "Duplicate keys"
 
-    If `object` already has more than one member with `key` (2.), `value` is assigned to the *last* one -- the member
-    [`operator[]`](../basic_json_view/operator%5B%5D.md), [`at`](../basic_json_view/at.md), and
-    [`find`](../basic_json_view/find.md) return for reading, so that a view taken from `object["key"]` before the call
-    shows `value` afterward -- and every other member with the same key is removed. The key stays at the
-    position of its *first* occurrence, where [`materialize()`](../basic_json_view/materialize.md) puts it as well. A
-    lookup, an iteration, and `materialize()` of `object` afterward therefore all agree on a single member for `key`. See the
-    [Notes on duplicate keys](../basic_json_view/operator%5B%5D.md#notes) of `operator[]`.
+    If `object` already has more than one member with `key` (2.), the *first* one is assigned `value` and every
+    later member with the same key is removed -- so that a lookup, an iteration, and
+    [`materialize()`](../basic_json_view/materialize.md) of `object` afterward all agree on a single value for
+    `key`, the same way [`operator[]`](../basic_json_view/operator%5B%5D.md) already picks the first occurrence of a
+    duplicate key for reading. See the [Notes on duplicate keys](../basic_json_view/operator%5B%5D.md#notes) of
+    `operator[]`.
 
 Setting a member (2.) or an element (3., through 4.) of an array or object whose elements have not been edited
 before switches it from its parsed layout to a growable block holding links to its elements; a later

docs/mkdocs/docs/api/basic_json_view/at.md

@@ -15,7 +15,7 @@ basic_json_view at(IntegerType idx) const;
 basic_json_view at(const json_pointer& ptr) const;
 ```
 
-1. Returns the value of the object member with key `key` -- the last one, should the key occur more than once (see
+1. Returns the value of the object member with key `key` -- the first one, should the key occur more than once (see
    [Notes on duplicate keys](operator[].md#notes)).
 2. Returns the array element at index `idx`. The template accepts every integer type except `#!cpp bool` and
    `#!cpp std::size_t` and forwards to the `size_type` overload, as for [`operator[]`](operator[].md); a negative
@@ -35,7 +35,7 @@ basic_json_view at(const json_pointer& ptr) const;
 
 ## Return value
 
-1. the value of the last member with key `key`
+1. the value of the first member with key `key`
 2. the element at index `idx`
 3. the value `ptr` resolves to, starting at this value
 
@@ -75,7 +75,7 @@ None of these exceptions carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics
 ## Complexity
 
 1. Linear in the number of members: as for [`ordered_json`](../ordered_json.md), members are compared one after
-   another, in document order, scanning all of them, since the last match is wanted. Each comparison first checks the
+   another, in document order, stopping at the first match. Each comparison first checks the
    key's length -- already known from the index, without reading the key bytes -- before comparing its content.
    Objects with 128 or more members get a hash index while parsing, so that a lookup in them takes constant time
    on average.
@@ -86,6 +86,13 @@ None of these exceptions carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics
 
 ## Notes
 
+!!! warning "Duplicate keys: the first member wins"
+
+    If the source text repeats a key, this resolves to the *first* member with it, not to the last one that
+    [`materialize()`](materialize.md) and `parse()` keep. See the [Notes on duplicate keys](operator[].md#notes) of
+    `operator[]` and [Duplicate keys](../../features/json_view.md#duplicate-keys) for the reasons and for how to get
+    the last value.
+
 Unlike [`operator[]`](operator[].md), which returns a [discarded](is_discarded.md) view for a missing key or an
 out-of-range index, `at` always throws -- exactly as `BasicJsonType::at` does, and with the same messages, so
 existing error handling written against `BasicJsonType::at` keeps working unchanged when switched to a view. This

docs/mkdocs/docs/api/basic_json_view/begin.md

@@ -24,7 +24,7 @@ Constant.
 
 For an object, iteration visits **every** member, including all occurrences of a duplicate key -- unlike
 [`operator[]`](operator[].md), [`at`](at.md), [`find`](find.md), [`contains`](contains.md), and [`count`](count.md),
-which all resolve to the *last* member with a given key. See the
+which all resolve to the *first* member with a given key. See the
 [Notes on duplicate keys](operator[].md#notes) of `operator[]`.
 
 Because objects are iterated in document order rather than sorted by key, the order seen here can differ from what

docs/mkdocs/docs/api/basic_json_view/contains.md

@@ -33,7 +33,7 @@ No-throw guarantee: this function never throws exceptions.
 ## Complexity
 
 1. Linear in the number of members: as for [`ordered_json`](../ordered_json.md), members are compared one after
-   another, in document order, scanning all of them, since the last match is wanted. Each comparison first checks the
+   another, in document order, stopping at the first match. Each comparison first checks the
    key's length -- already known from the index, without reading the key bytes -- before comparing its content.
    Objects with 128 or more members get a hash index while parsing, so that a lookup in them takes constant time
    on average.
@@ -43,6 +43,13 @@ No-throw guarantee: this function never throws exceptions.
 
 ## Notes
 
+!!! warning "Duplicate keys: the first member wins"
+
+    If the source text repeats a key, this resolves to the *first* member with it, not to the last one that
+    [`materialize()`](materialize.md) and `parse()` keep. See the [Notes on duplicate keys](operator[].md#notes) of
+    `operator[]` and [Duplicate keys](../../features/json_view.md#duplicate-keys) for the reasons and for how to get
+    the last value.
+
 Overload 1 always returns `#!cpp false` when the value is not an object -- including a [discarded](is_discarded.md)
 view.
 

docs/mkdocs/docs/api/basic_json_view/count.md

@@ -24,13 +24,20 @@ No-throw guarantee: this function never throws exceptions.
 ## Complexity
 
 Linear in the number of members: as for [`ordered_json`](../ordered_json.md), members are compared one after another,
-in document order, scanning all of them, since the last match is wanted. Each comparison first checks the key's length
+in document order, stopping at the first match. Each comparison first checks the key's length
 -- already known from the index, without reading the key bytes -- before comparing its content.
 Objects with 128 or more members get a hash index while parsing, so that a lookup in them takes constant time on
 average.
 
 ## Notes
 
+!!! warning "Duplicate keys: the first member wins"
+
+    If the source text repeats a key, this resolves to the *first* member with it, not to the last one that
+    [`materialize()`](materialize.md) and `parse()` keep. See the [Notes on duplicate keys](operator[].md#notes) of
+    `operator[]` and [Duplicate keys](../../features/json_view.md#duplicate-keys) for the reasons and for how to get
+    the last value.
+
 This method always returns `#!cpp 0` when the value is not an object -- including a [discarded](is_discarded.md)
 view.
 
@@ -38,7 +45,7 @@ Unlike [`BasicJsonType::count()`](../basic_json/count.md), whose return value ca
 an `ObjectType` that allows multiple entries per key, `count()` here never does: it is exactly
 [`contains()`](contains.md) as `#!cpp 0`/`#!cpp 1`. This holds even if the source text has a duplicate key -- see the
 [Notes on duplicate keys](operator[].md#notes) of `operator[]` -- because a `#!cpp count() > 1` result would require
-counting every member with a matching key (the lookup functions resolve to the *last* one).
+counting every member with a matching key (the lookup functions resolve to the *first* one).
 
 ## Examples
 

docs/mkdocs/docs/api/basic_json_view/find.md

@@ -6,7 +6,7 @@ iterator find(const char* key) const;
 iterator find(const string_t& key) const;
 ```
 
-Finds a member with key `key` -- the last one, should the key occur more than once (see
+Finds a member with key `key` -- the first one, should the key occur more than once (see
 [Notes on duplicate keys](operator[].md#notes)). If the value is not an object, or no member has this key,
 [`end()`](end.md) is returned.
 
@@ -26,13 +26,20 @@ No-throw guarantee: this function never throws exceptions.
 ## Complexity
 
 Linear in the number of members: as for [`ordered_json`](../ordered_json.md), members are compared one after another,
-in document order, scanning all of them, since the last match is wanted. Each comparison first checks the key's length
+in document order, stopping at the first match. Each comparison first checks the key's length
 -- already known from the index, without reading the key bytes -- before comparing its content.
 Objects with 128 or more members get a hash index while parsing, so that a lookup in them takes constant time on
 average.
 
 ## Notes
 
+!!! warning "Duplicate keys: the first member wins"
+
+    If the source text repeats a key, this resolves to the *first* member with it, not to the last one that
+    [`materialize()`](materialize.md) and `parse()` keep. See the [Notes on duplicate keys](operator[].md#notes) of
+    `operator[]` and [Duplicate keys](../../features/json_view.md#duplicate-keys) for the reasons and for how to get
+    the last value.
+
 Unlike [`BasicJsonType::find`](../basic_json/find.md), which always returns `#!cpp end()` for a non-object type, this
 also does so for a [discarded](is_discarded.md) view -- there is no separate "invalid" iterator to return.
 

docs/mkdocs/docs/api/basic_json_view/get.md

@@ -87,9 +87,9 @@ exception thrown while converting through `materialize()` (the last bullet) is d
 !!! info "Duplicate keys"
 
     `#!cpp std::map`/`#!cpp std::unordered_map` conversions keep the *last* value of a repeated key, like
-    [`materialize()`](materialize.md) and [`BasicJsonType::parse()`](../basic_json/parse.md) do. This is the member
-    [`operator[]`](operator[].md)/[`at`](at.md)/[`find`](find.md)/[`contains`](contains.md) resolve to, too (see the
-    [Notes on duplicate keys](operator[].md#notes)).
+    [`materialize()`](materialize.md) and [`BasicJsonType::parse()`](../basic_json/parse.md) do. This is the opposite
+    of [`operator[]`](operator[].md)/[`at`](at.md)/[`find`](find.md)/[`contains`](contains.md), which resolve to the
+    *first* occurrence (see the [Notes on duplicate keys](operator[].md#notes)).
 
 !!! info "No pointers, references, or implicit conversion"
 

docs/mkdocs/docs/api/basic_json_view/items.md

@@ -48,7 +48,7 @@ Constant.
 
 As for [`begin()`](begin.md)/[`end()`](end.md), `items()` visits **every** member of an object, including all
 occurrences of a duplicate key -- unlike [`operator[]`](operator[].md), [`at`](at.md), [`find`](find.md),
-[`contains`](contains.md), and [`count`](count.md), which resolve to the *last* member with a given key. See the
+[`contains`](contains.md), and [`count`](count.md), which resolve to the *first* member with a given key. See the
 [Notes on duplicate keys](operator[].md#notes) of `operator[]`.
 
 !!! danger "Lifetime issues"
@@ -63,8 +63,8 @@ occurrences of a duplicate key -- unlike [`operator[]`](operator[].md), [`at`](a
 
     The example below shows a settings object whose source text records every update to a key as a duplicate
     member, in the order they happened. `items()` walks all of them, so the update history is visible, while
-    [`operator[]`](operator[].md) sees the *last* one, and so does [`materialize()`](materialize.md) -- like
-    [`BasicJsonType::parse()`](../basic_json/parse.md).
+    [`operator[]`](operator[].md) only ever sees the *first* one and [`materialize()`](materialize.md) -- like
+    [`BasicJsonType::parse()`](../basic_json/parse.md) -- keeps only the *last*.
 
     ```cpp
     --8<-- "examples/basic_json_view__items.cpp"

docs/mkdocs/docs/api/basic_json_view/operator[].md

@@ -15,7 +15,7 @@ basic_json_view operator[](IntegerType idx) const;
 basic_json_view operator[](const json_pointer& ptr) const;
 ```
 
-1. Returns the value of the object member with key `key` -- the last one, should the key occur more than once (see
+1. Returns the value of the object member with key `key` -- the first one, should the key occur more than once (see
    the [Notes](#notes) below) -- or a [discarded](is_discarded.md) view if there is no such member.
 2. Returns the array element at index `idx`, or a [discarded](is_discarded.md) view if `idx` is out of range. The
    template accepts every integer type except `#!cpp bool` and `#!cpp std::size_t` (`#!cpp int`, `#!cpp unsigned`,
@@ -38,7 +38,7 @@ basic_json_view operator[](const json_pointer& ptr) const;
 
 ## Return value
 
-1. the value of the last member with key `key`, or a discarded view if no member has this key (or if this view is
+1. the value of the first member with key `key`, or a discarded view if no member has this key (or if this view is
    [discarded](is_discarded.md))
 2. the element at index `idx`, or a discarded view if `#!cpp idx >= size()` or `idx` is negative (or if this view is
    [discarded](is_discarded.md))
@@ -80,7 +80,7 @@ None of these exceptions carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics
 ## Complexity
 
 1. Linear in the number of members: as for [`ordered_json`](../ordered_json.md), members are compared one after
-   another, in document order, scanning all of them, since the last match is wanted. Each comparison first checks the
+   another, in document order, stopping at the first match. Each comparison first checks the
    key's length -- already known from the index, without reading the key bytes -- before comparing its content, so a
    key of a different length than `key` is rejected without touching the source text.
    Objects with 128 or more members get a hash index while parsing, so that a lookup in them takes constant time
@@ -107,16 +107,19 @@ document.
     key on an array or a primitive, or an index on an object or a primitive, is `type_error.305` as for
     `BasicJsonType`. [`at`](at.md) still throws for a discarded view, as it does for a missing key.
 
-!!! info "Duplicate keys"
+!!! warning "Duplicate keys: the first member wins"
 
     If the source text has an object with a duplicate key, `#!cpp operator[]` (and [`at`](at.md), [`find`](find.md),
     [`contains`](contains.md), [`count`](count.md), [`value`](value.md), and JSON pointer resolution) all resolve to
-    the *last* member with that key. This is the member [`materialize()`](materialize.md) (and
-    [`BasicJsonType::parse()`](../basic_json/parse.md)) keeps, so a lookup in the view and in the materialized value
-    agree. [`begin()`](begin.md)/[`end()`](end.md) and [`items()`](items.md) iterate over *all* members, including
-    duplicates, in document order. A lookup in an object without a hash index scans all members for this: it cannot
-    stop at the first match. The hash index of a larger object (128 members or more) leads to the last member of a key
-    as well. See the example below and [`size()`](size.md#notes).
+    the *first* member with that key, because a lookup can stop as soon as it finds a match. This is different from
+    [`materialize()`](materialize.md) (and [`BasicJsonType::parse()`](../basic_json/parse.md)), which replay every
+    member in order and so keep the *last* value for a repeated key, so `#!cpp v["a"]` and
+    `#!cpp v.materialize()["a"]` can differ. To get the value `parse()` would give, use
+    [`materialize()`](materialize.md) or iterate the members with [`items()`](items.md) and keep the last match.
+    The hash index of a larger object (128 members or more) leads to the first member of a key as well.
+    [`begin()`](begin.md)/[`end()`](end.md) and [`items()`](items.md) iterate over *all* members, including
+    duplicates, in document order. See [Duplicate keys](../../features/json_view.md#duplicate-keys) and
+    [`size()`](size.md#notes).
 
 !!! info "JSON pointer resolution"
 

docs/mkdocs/docs/api/basic_json_view/operator_eq.md

@@ -18,6 +18,8 @@ bool operator==(const BasicJsonType& lhs, const basic_json_view& rhs);
 Neither overload builds a `BasicJsonType` value for a view to do the comparison (see [Notes](#notes) below). Numbers
 compare by value across their types (`#!cpp 1 == 1.0`), and an object compares by its members, with duplicate keys
 resolved exactly as `parse()` resolves them -- the last value, at the position of the first occurrence of the key.
+This is not what [`operator[]`](operator[].md) and the other lookups return for a duplicate key: they find the *first*
+member (see [Duplicate keys](../../features/json_view.md#duplicate-keys)).
 
 ## Parameters
 

docs/mkdocs/docs/api/basic_json_view/value.md

@@ -12,7 +12,7 @@ T value(const json_pointer& ptr, const T& default_value) const;
 string_t value(const json_pointer& ptr, const char* default_value) const;
 ```
 
-1. Returns the value of the object member with key `key` -- the last one, should the key occur more than once (see
+1. Returns the value of the object member with key `key` -- the first one, should the key occur more than once (see
    [Notes on duplicate keys](operator[].md#notes)) -- converted to `T`, or `default_value` if there is no such member.
 2. Returns the value a JSON pointer `ptr` refers to, starting at this value, converted to `T`, or `default_value` if
    `ptr` cannot be resolved.
@@ -39,7 +39,7 @@ equivalent) deduce `string_t`, not `const char*`, for their return type and for
 
 ## Return value
 
-1. the last member with key `key`, converted to `T`, or `default_value`
+1. the first member with key `key`, converted to `T`, or `default_value`
 2. the value `ptr` resolves to, converted to `T`, or `default_value`
 
 ## Exception safety
@@ -68,7 +68,7 @@ None of these exceptions carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics
 ## Complexity
 
 1. Linear in the number of members: as for [`operator[]`](operator[].md#complexity), members are compared one after
-   another, in document order, scanning all of them, since the last match is wanted. Plus the complexity of converting
+   another, in document order, stopping at the first match. Plus the complexity of converting
    the found member to `T` (see [`get`](get.md)).
    Objects with 128 or more members get a hash index while parsing, so that a lookup in them takes constant time
    on average.
@@ -79,6 +79,13 @@ None of these exceptions carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics
 
 ## Notes
 
+!!! warning "Duplicate keys: the first member wins"
+
+    If the source text repeats a key, this resolves to the *first* member with it, not to the last one that
+    [`materialize()`](materialize.md) and `parse()` keep. See the [Notes on duplicate keys](operator[].md#notes) of
+    `operator[]` and [Duplicate keys](../../features/json_view.md#duplicate-keys) for the reasons and for how to get
+    the last value.
+
 !!! info "Differences to `at` and `operator[]`"
 
     Unlike [`at`](at.md), this function does not throw if `key`/`ptr` resolves to no value. Unlike

docs/mkdocs/docs/examples/basic_json_view__items.cpp

@@ -7,8 +7,8 @@ int main()
 {
     // a settings object whose source text records every update to a key as
     // a duplicate member. items() visits all of them, in document order, so
-    // the update history is visible; operator[] and materialize() -- like
-    // basic_json::parse() -- see the last one
+    // the update history is visible; operator[] only ever sees the first
+    // one, and materialize() -- like basic_json::parse() -- keeps the last
     json_document updates = json_document::parse(R"({"retries": 1, "timeout": 30, "retries": 5})");
     const auto settings = updates.root();
 
@@ -17,6 +17,6 @@ int main()
         std::cout << item.key() << '=' << item.value().materialize().dump() << '\n';
     }
 
-    std::cout << "last \"retries\" seen by operator[]: " << settings["retries"].materialize().dump() << '\n';
+    std::cout << "first \"retries\" seen by operator[]: " << settings["retries"].materialize().dump() << '\n';
     std::cout << "last \"retries\" kept by materialize(): " << settings.materialize()["retries"].dump() << '\n';
 }

docs/mkdocs/docs/examples/basic_json_view__items.output

@@ -1,5 +1,5 @@
 retries=1
 timeout=30
 retries=5
-last "retries" seen by operator[]: 5
+first "retries" seen by operator[]: 1
 last "retries" kept by materialize(): 5

docs/mkdocs/docs/features/json_view.md

@@ -134,17 +134,8 @@ document: `#!cpp auto v = json_document::parse(text).root();` does not compile.
   `operator[]` on a discarded view returns a discarded view without throwing: `#!cpp v["a"]["b"][0]` can be tested
   once at the end. Type errors on values that exist (a key on an array, an index on an object) still throw, and
   [`at`](../api/basic_json_view/at.md) throws for every missing value.
-- **Duplicate keys are visible.** If an object in the source text repeats a key,
-  [`begin()`](../api/basic_json_view/begin.md)/[`end()`](../api/basic_json_view/end.md) and
-  [`items()`](../api/basic_json_view/items.md) visit *every* occurrence (and [`size()`](../api/basic_json_view/size.md)
-  counts all of them), while [`operator[]`](../api/basic_json_view/operator%5B%5D.md),
-  [`at`](../api/basic_json_view/at.md), [`find`](../api/basic_json_view/find.md),
-  [`contains`](../api/basic_json_view/contains.md), and [`count`](../api/basic_json_view/count.md) resolve to the
-  *last* occurrence -- the one `basic_json::parse()` (and so
-  [`materialize()`](../api/basic_json_view/materialize.md)) keeps for a repeated key -- which makes a lookup scan all
-  members instead of stopping at a match (objects with 128 members or more get a hash index that leads to the last
-  occurrence directly).
-  See the [Notes on duplicate keys](../api/basic_json_view/operator%5B%5D.md#notes) of `operator[]`.
+- **Duplicate keys: lookups find the first member, `parse()` keeps the last.** See [Duplicate keys](#duplicate-keys)
+  below.
 - **No [`JSON_DIAGNOSTICS`](../api/macros/json_diagnostics.md) path.** Exceptions thrown by `basic_json_view`'s own
   element access and lookup functions never carry the JSON Pointer path `JSON_DIAGNOSTICS` would otherwise add: the
   view has no `basic_json` value to point at, so the exception is created without one, regardless of how
@@ -156,6 +147,52 @@ document: `#!cpp auto v = json_document::parse(text).root();` does not compile.
   equal values for them, without ever building a tree to do it. For ordering, too,
   [`materialize()`](../api/basic_json_view/materialize.md) is the way to get a value you can compare.
 
+## Duplicate keys
+
+!!! warning "A lookup in a view and in the parsed value can give different answers"
+
+    If an object in the source text repeats a key, [`operator[]`](../api/basic_json_view/operator%5B%5D.md),
+    [`at`](../api/basic_json_view/at.md), [`find`](../api/basic_json_view/find.md),
+    [`value`](../api/basic_json_view/value.md), [`contains`](../api/basic_json_view/contains.md),
+    [`count`](../api/basic_json_view/count.md), and JSON pointer resolution all return the **first** member with that
+    key. `basic_json::parse()` instead keeps the **last** value of a repeated key, so for `#!cpp {"a":1,"a":2}`,
+    `#!cpp view["a"]` is `#!cpp 1` while `#!cpp parse(text)["a"]` is `#!cpp 2`.
+
+[`begin()`](../api/basic_json_view/begin.md)/[`end()`](../api/basic_json_view/end.md) and
+[`items()`](../api/basic_json_view/items.md) visit *every* occurrence, in document order, and
+[`size()`](../api/basic_json_view/size.md) counts all of them, so `#!cpp view.size()` can be larger than
+`#!cpp view.materialize().size()`.
+
+**Why the first?** A lookup can stop as soon as it finds a match. Returning the last member would force every lookup
+to scan all members of the object, even when the key is found at the very first one: this made lookups in small
+objects 1.6 to 3.4 times slower. The hash index of objects with 128 members or more leads to the first member of a
+key as well. Other zero-copy parsers that index the source text, such as yyjson and simdjson, also return the first
+member. RFC 8259 only says that names within an object SHOULD be unique and that the behavior of a receiver that sees
+duplicates is unpredictable, so neither choice is wrong.
+
+**What stays the same as `parse()`?** [`materialize()`](../api/basic_json_view/materialize.md) and
+[`get<std::map<...>>()`](../api/basic_json_view/get.md) replay every member in order, so they keep the *last* value
+exactly like `#!cpp basic_json::parse()` (at the position of the first occurrence of the key, for an
+[`ordered_json`](../api/ordered_json.md)).
+
+**How do I get the value `parse()` would give?** Either call `#!cpp view.materialize()` and look the key up in the
+result, or iterate the members and keep the last match:
+
+```cpp
+// the last member with the key "a", as parse() would keep it
+json_view last;
+for (const auto item : view.items())
+{
+    if (item.key() == "a")
+    {
+        last = item.value();
+    }
+}
+```
+
+If you cannot trust the source text to have unique keys, check [`size()`](../api/basic_json_view/size.md) against the
+number of distinct keys, or reject duplicates when iterating.
+
 ## Getting values out without copying
 
 [`get<T>()`](../api/basic_json_view/get.md) converts many `T` directly from the flat index, without ever building a
@@ -185,8 +222,8 @@ Both results are only valid as long as the view -- and, for a string with no esc
 [`dump()`](../api/basic_json_view/dump.md) serializes a view directly from the flat index, without ever building a
 `basic_json` value. An object's members are written in document order, not sorted by key, and *every* occurrence of a
 repeated key is written, not only the last one -- the same two ways [iteration](#what-is-different) already differs
-from a [`materialize()`](../api/basic_json_view/materialize.md)d value, see above. `#!cpp materialize().dump()` gives
-a different result in both respects for a `json_view`.
+from a [`materialize()`](../api/basic_json_view/materialize.md)d value, see [Duplicate keys](#duplicate-keys).
+`#!cpp materialize().dump()` gives a different result in both respects for a `json_view`.
 
 By default, numbers are written the way [`basic_json::dump()`](../api/basic_json/dump.md) would.
 [`number_format::source`](../api/basic_json_view/number_format.md) instead copies every number exactly as it was

docs/mkdocs/docs/home/architecture.md

@@ -215,7 +215,7 @@ packet-beta
 - **Large objects** (128 members or more) get a hash index after parsing
   ([`detail/view/object_index.hpp`](https://github.com/nlohmann/json/blob/develop/include/nlohmann/detail/view/object_index.hpp)):
   an open-addressing table whose slots hold the distance from the object's node to a key's node, so that a lookup does
-  not compare every key. Of duplicate keys the table leads to the last, as a linear search does. The object's `extra`
+  not compare every key. Of duplicate keys the table leads to the first, as a linear search does. The object's `extra`
   holds the number of its table. Only 65,535 tables fit into `extra`; objects beyond them are searched linearly.
 
 For example, `#!json {"a": [1, 2.5]}` becomes five nodes. Each node's elements follow it, and `next` leads from an

include/nlohmann/detail/view/edit.hpp

@@ -152,23 +152,25 @@ class editor
         {
             become_empty(o, value_t::object);
         }
-        // an existing member: assign the one that lookups find (the last
-        // one, should the key occur more than once), and drop the others, so
-        // that lookups, iteration, and materialize() agree. The key stays at
-        // the position of its first occurrence, as materialize() puts it.
+        // an existing member: assign it (and drop later duplicates, so that
+        // lookups, iteration, and materialize() agree)
         node* slot = nullptr;
-        std::size_t matches = 0;
+        bool duplicates = false;
         for (const node* k = nav::first(m_doc, o), *end = nav::end(m_doc, o); k != end; k = document_data::after(k + 1))
         {
             if (key_equals(*k, key))
             {
+                if (slot != nullptr)
+                {
+                    duplicates = true;
+                    break;
+                }
                 slot = const_cast<node*>(nav::value(k + 1)); // NOLINT(cppcoreguidelines-pro-type-const-cast): the nodes belong to this document
-                ++matches;
             }
         }
         if (slot != nullptr)
         {
-            if (matches > 1)
+            if (duplicates)
             {
                 erase_members(o, key, true);
             }
@@ -314,38 +316,27 @@ class editor
         return k.len == key.size() && (key.size() == 0 || std::memcmp(m_doc.str(k), key.data(), key.size()) == 0);
     }
 
-    /// Remove the members with this key from an object: all of them, or all
-    /// but one. That one stays where the first occurrence is, but holds the
-    /// value of the last (the one that lookups find, which views may refer to).
-    std::size_t erase_members(node* o, string_view_t key, bool keep_one)
+    /// remove the members with this key (all, or all but the first) from an object
+    std::size_t erase_members(node* o, string_view_t key, bool keep_first)
     {
         node* const h = block_of(m_doc, o, 0);
-        node last_value{}; // the entry of the value of the last member
-        node* const end = h + h->next;
-        if (keep_one)
-        {
-            for (node* r = h + 1; r != end; r += 2)
-            {
-                if (key_equals(*r, key))
-                {
-                    last_value = r[1];
-                }
-            }
-        }
         node* w = h + 1;
         std::size_t erased = 0;
         bool kept = false;
-        for (node* r = h + 1; r != end; r += 2)
+        for (node* r = h + 1, *end = h + h->next; r != end; r += 2)
         {
             const bool match = key_equals(*r, key);
-            if (match && (kept || !keep_one))
+            if (match && (kept || !keep_first))
             {
                 ++erased;
                 continue;
             }
-            w[0] = r[0];
-            w[1] = match ? last_value : r[1];
             kept = kept || match;
+            if (w != r)
+            {
+                w[0] = r[0];
+                w[1] = r[1];
+            }
             w += 2;
         }
         h->next = static_cast<std::uint32_t>(w - h);

include/nlohmann/detail/view/lookup.hpp

@@ -84,9 +84,10 @@ class short_key
     std::uint64_t m_b = 0;
 };
 
-/// the key node of the last member of an object with the given key, or
-/// nullptr (the last one, as materialize() and parse() keep it); most keys are
-/// rejected by their length, from the index alone
+/// the key node of the first member of an object with the given key, or
+/// nullptr (the search stops at the first match; materialize() and parse()
+/// keep the last value of a duplicate key instead); most keys are rejected by
+/// their length, from the index alone
 template<bool Editable>
 const node* find_member(const document_data& d, const node* object, const char* key, std::size_t n) noexcept
 {
@@ -97,7 +98,6 @@ const node* find_member(const document_data& d, const node* object, const char*
     }
     const node* const end = nav::end(d, object);
     const auto* const k = reinterpret_cast<const unsigned char*>(key); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
-    const node* last = nullptr;
     if (NLOHMANN_VIEW_LIKELY(n <= 16))
     {
         const short_key probe(k, n);
@@ -105,19 +105,19 @@ const node* find_member(const document_data& d, const node* object, const char*
         {
             if (m->len == n && probe.matches(reinterpret_cast<const unsigned char*>(d.str(*m)))) // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
             {
-                last = m;
+                return m;
             }
         }
-        return last;
+        return nullptr;
     }
     for (const node* m = nav::first(d, object); m != end; m = document_data::after(m + 1))
     {
         if (m->len == n && std::memcmp(d.str(*m), key, n) == 0)
         {
-            last = m;
+            return m;
         }
     }
-    return last;
+    return nullptr;
 }
 
 /// whether an integer type is accepted as an array index by the view's

include/nlohmann/detail/view/object_index.hpp

@@ -22,8 +22,8 @@
 // large objects). An object with document_data::index_min_members members or
 // more gets an open-addressing table after parsing; its node stores the
 // number of the table (1-based) in `extra`. A slot holds the offset of a key
-// node from its object node (0: empty). Of duplicate keys, the last is kept,
-// as for the linear search, and as basic_json::parse() does.
+// node from its object node (0: empty). Of duplicate keys, the first is kept,
+// as for the linear search.
 //
 // The hash is not seeded, so keys chosen to collide could make the build
 // quadratic. A key therefore sits at most index_max_displacement slots away
@@ -92,8 +92,7 @@ inline void build_object_index(document_data& d, node* obj)
             const node* const other = obj + slots[i];
             if (other->len == k->len && (k->len == 0 || std::memcmp(d.str(*other), key, k->len) == 0))
             {
-                duplicate = true; // keep the last: the key's slot now leads to this member
-                slots[i] = static_cast<std::uint32_t>(k - obj);
+                duplicate = true; // keep the first
                 break;
             }
             if (++distance > index_max_displacement)
@@ -130,7 +129,7 @@ inline void build_object_indexes(document_data& d)
     }
 }
 
-/// the key node of the last member with this key of an indexed object, or
+/// the key node of the first member with this key of an indexed object, or
 /// nullptr
 inline const node* find_indexed(const document_data& d, const node* obj, const char* key, std::size_t n) noexcept
 {

include/nlohmann/detail/view/serializer.hpp

@@ -50,11 +50,6 @@ class output_buffer
     {
         const auto size = static_cast<std::size_t>(m_pos - m_out.data());
         m_out.resize(size);
-        // do not keep a buffer that was sized for a much larger output
-        if (m_out.capacity() > 1024 && m_out.capacity() / 2 > size)
-        {
-            m_out.shrink_to_fit();
-        }
     }
 
     NLOHMANN_VIEW_ALWAYS_INLINE void reserve(std::size_t n)

include/nlohmann/json_view.hpp

@@ -243,7 +243,7 @@ class basic_json_view
     // element access //
     ////////////////////
 
-    /// the value of the member with this key (the last one, should the key
+    /// the value of the member with this key (the first one, should the key
     /// occur more than once); a discarded view if there is none, or if this
     /// is a discarded view (so that v["a"]["b"] is safe). Throws type_error.305
     /// if this is any other value but an object.
@@ -307,7 +307,7 @@ class basic_json_view
         return detail::view::resolve_pointer(*this, detail::json_pointer_access::reference_tokens(ptr), detail::view::pointer_mode::unchecked);
     }
 
-    /// the value of the member with this key (the last one, should the key
+    /// the value of the member with this key (the first one, should the key
     /// occur more than once). Throws type_error.304 if this is not an object,
     /// and out_of_range.403 if there is no such member.
     basic_json_view at(string_view_t key) const
@@ -365,7 +365,7 @@ class basic_json_view
     }
 
     /// the member with this key converted to T, or the default value if there
-    /// is no such member (the last one, should the key occur more than
+    /// is no such member (the first one, should the key occur more than
     /// once). Throws type_error.306 if this is not an object.
     template < typename T, typename std::enable_if < !std::is_same<typename std::decay<T>::type, const char*>::value, int >::type = 0 >
     T value(string_view_t key, const T& default_value) const
@@ -430,7 +430,7 @@ class basic_json_view
     // lookup //
     ////////////
 
-    /// an iterator to the member with this key (the last one, should the
+    /// an iterator to the member with this key (the first one, should the
     /// key occur more than once), or end(); end() also for non-objects
     iterator find(string_view_t key) const
     {
@@ -765,7 +765,7 @@ class basic_json_view
         return (std::min)(m_doc->size - m_node->off, static_cast<std::size_t>(1024) + nodes * 16);
     }
 
-    /// the value of the last member with this key, or a discarded view
+    /// the value of the first member with this key, or a discarded view
     /// (object required)
     NLOHMANN_VIEW_ALWAYS_INLINE basic_json_view lookup(string_view_t key) const noexcept
     {

single_include/nlohmann/json_view.hpp

@@ -2939,8 +2939,8 @@ NLOHMANN_JSON_NAMESPACE_END
 // large objects). An object with document_data::index_min_members members or
 // more gets an open-addressing table after parsing; its node stores the
 // number of the table (1-based) in `extra`. A slot holds the offset of a key
-// node from its object node (0: empty). Of duplicate keys, the last is kept,
-// as for the linear search, and as basic_json::parse() does.
+// node from its object node (0: empty). Of duplicate keys, the first is kept,
+// as for the linear search.
 //
 // The hash is not seeded, so keys chosen to collide could make the build
 // quadratic. A key therefore sits at most index_max_displacement slots away
@@ -3009,8 +3009,7 @@ inline void build_object_index(document_data& d, node* obj)
             const node* const other = obj + slots[i];
             if (other->len == k->len && (k->len == 0 || std::memcmp(d.str(*other), key, k->len) == 0))
             {
-                duplicate = true; // keep the last: the key's slot now leads to this member
-                slots[i] = static_cast<std::uint32_t>(k - obj);
+                duplicate = true; // keep the first
                 break;
             }
             if (++distance > index_max_displacement)
@@ -3047,7 +3046,7 @@ inline void build_object_indexes(document_data& d)
     }
 }
 
-/// the key node of the last member with this key of an indexed object, or
+/// the key node of the first member with this key of an indexed object, or
 /// nullptr
 inline const node* find_indexed(const document_data& d, const node* obj, const char* key, std::size_t n) noexcept
 {
@@ -3141,9 +3140,10 @@ class short_key
     std::uint64_t m_b = 0;
 };
 
-/// the key node of the last member of an object with the given key, or
-/// nullptr (the last one, as materialize() and parse() keep it); most keys are
-/// rejected by their length, from the index alone
+/// the key node of the first member of an object with the given key, or
+/// nullptr (the search stops at the first match; materialize() and parse()
+/// keep the last value of a duplicate key instead); most keys are rejected by
+/// their length, from the index alone
 template<bool Editable>
 const node* find_member(const document_data& d, const node* object, const char* key, std::size_t n) noexcept
 {
@@ -3154,7 +3154,6 @@ const node* find_member(const document_data& d, const node* object, const char*
     }
     const node* const end = nav::end(d, object);
     const auto* const k = reinterpret_cast<const unsigned char*>(key); // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
-    const node* last = nullptr;
     if (NLOHMANN_VIEW_LIKELY(n <= 16))
     {
         const short_key probe(k, n);
@@ -3162,19 +3161,19 @@ const node* find_member(const document_data& d, const node* object, const char*
         {
             if (m->len == n && probe.matches(reinterpret_cast<const unsigned char*>(d.str(*m)))) // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
             {
-                last = m;
+                return m;
             }
         }
-        return last;
+        return nullptr;
     }
     for (const node* m = nav::first(d, object); m != end; m = document_data::after(m + 1))
     {
         if (m->len == n && std::memcmp(d.str(*m), key, n) == 0)
         {
-            last = m;
+            return m;
         }
     }
-    return last;
+    return nullptr;
 }
 
 /// whether an integer type is accepted as an array index by the view's
@@ -3361,23 +3360,25 @@ class editor
         {
             become_empty(o, value_t::object);
         }
-        // an existing member: assign the one that lookups find (the last
-        // one, should the key occur more than once), and drop the others, so
-        // that lookups, iteration, and materialize() agree. The key stays at
-        // the position of its first occurrence, as materialize() puts it.
+        // an existing member: assign it (and drop later duplicates, so that
+        // lookups, iteration, and materialize() agree)
         node* slot = nullptr;
-        std::size_t matches = 0;
+        bool duplicates = false;
         for (const node* k = nav::first(m_doc, o), *end = nav::end(m_doc, o); k != end; k = document_data::after(k + 1))
         {
             if (key_equals(*k, key))
             {
+                if (slot != nullptr)
+                {
+                    duplicates = true;
+                    break;
+                }
                 slot = const_cast<node*>(nav::value(k + 1)); // NOLINT(cppcoreguidelines-pro-type-const-cast): the nodes belong to this document
-                ++matches;
             }
         }
         if (slot != nullptr)
         {
-            if (matches > 1)
+            if (duplicates)
             {
                 erase_members(o, key, true);
             }
@@ -3523,38 +3524,27 @@ class editor
         return k.len == key.size() && (key.size() == 0 || std::memcmp(m_doc.str(k), key.data(), key.size()) == 0);
     }
 
-    /// Remove the members with this key from an object: all of them, or all
-    /// but one. That one stays where the first occurrence is, but holds the
-    /// value of the last (the one that lookups find, which views may refer to).
-    std::size_t erase_members(node* o, string_view_t key, bool keep_one)
+    /// remove the members with this key (all, or all but the first) from an object
+    std::size_t erase_members(node* o, string_view_t key, bool keep_first)
     {
         node* const h = block_of(m_doc, o, 0);
-        node last_value{}; // the entry of the value of the last member
-        node* const end = h + h->next;
-        if (keep_one)
-        {
-            for (node* r = h + 1; r != end; r += 2)
-            {
-                if (key_equals(*r, key))
-                {
-                    last_value = r[1];
-                }
-            }
-        }
         node* w = h + 1;
         std::size_t erased = 0;
         bool kept = false;
-        for (node* r = h + 1; r != end; r += 2)
+        for (node* r = h + 1, *end = h + h->next; r != end; r += 2)
         {
             const bool match = key_equals(*r, key);
-            if (match && (kept || !keep_one))
+            if (match && (kept || !keep_first))
             {
                 ++erased;
                 continue;
             }
-            w[0] = r[0];
-            w[1] = match ? last_value : r[1];
             kept = kept || match;
+            if (w != r)
+            {
+                w[0] = r[0];
+                w[1] = r[1];
+            }
             w += 2;
         }
         h->next = static_cast<std::uint32_t>(w - h);
@@ -5835,11 +5825,6 @@ class output_buffer
     {
         const auto size = static_cast<std::size_t>(m_pos - m_out.data());
         m_out.resize(size);
-        // do not keep a buffer that was sized for a much larger output
-        if (m_out.capacity() > 1024 && m_out.capacity() / 2 > size)
-        {
-            m_out.shrink_to_fit();
-        }
     }
 
     NLOHMANN_VIEW_ALWAYS_INLINE void reserve(std::size_t n)
@@ -7151,7 +7136,7 @@ class basic_json_view
     // element access //
     ////////////////////
 
-    /// the value of the member with this key (the last one, should the key
+    /// the value of the member with this key (the first one, should the key
     /// occur more than once); a discarded view if there is none, or if this
     /// is a discarded view (so that v["a"]["b"] is safe). Throws type_error.305
     /// if this is any other value but an object.
@@ -7215,7 +7200,7 @@ class basic_json_view
         return detail::view::resolve_pointer(*this, detail::json_pointer_access::reference_tokens(ptr), detail::view::pointer_mode::unchecked);
     }
 
-    /// the value of the member with this key (the last one, should the key
+    /// the value of the member with this key (the first one, should the key
     /// occur more than once). Throws type_error.304 if this is not an object,
     /// and out_of_range.403 if there is no such member.
     basic_json_view at(string_view_t key) const
@@ -7273,7 +7258,7 @@ class basic_json_view
     }
 
     /// the member with this key converted to T, or the default value if there
-    /// is no such member (the last one, should the key occur more than
+    /// is no such member (the first one, should the key occur more than
     /// once). Throws type_error.306 if this is not an object.
     template < typename T, typename std::enable_if < !std::is_same<typename std::decay<T>::type, const char*>::value, int >::type = 0 >
     T value(string_view_t key, const T& default_value) const
@@ -7338,7 +7323,7 @@ class basic_json_view
     // lookup //
     ////////////
 
-    /// an iterator to the member with this key (the last one, should the
+    /// an iterator to the member with this key (the first one, should the
     /// key occur more than once), or end(); end() also for non-objects
     iterator find(string_view_t key) const
     {
@@ -7673,7 +7658,7 @@ class basic_json_view
         return (std::min)(m_doc->size - m_node->off, static_cast<std::size_t>(1024) + nodes * 16);
     }
 
-    /// the value of the last member with this key, or a discarded view
+    /// the value of the first member with this key, or a discarded view
     /// (object required)
     NLOHMANN_VIEW_ALWAYS_INLINE basic_json_view lookup(string_view_t key) const noexcept
     {

tests/src/unit-json_view.cpp

@@ -577,7 +577,7 @@ std::string exception_of(F f)
 
 // compares a view with the ordered_json value materialize() gives for it:
 // types, sizes, elements and members (by index, key, and iteration), in
-// document order; duplicate keys are found as their last occurrence
+// document order; duplicate keys are found as their first occurrence
 void check_access(const ordered_json_view& v, const ordered_json& j)
 {
     REQUIRE(v.type() == j.type());
@@ -618,23 +618,13 @@ void check_access(const ordered_json_view& v, const ordered_json& j)
             const std::string key(it.key().data(), it.key().size());
             CHECK(v.contains(key));
             CHECK(v.count(key) == 1);
-            if (std::find(keys.begin(), keys.end(), key) == keys.end())
-            {
-                keys.push_back(key);
-            }
-            // lookups find the last member with the key, which is this one if
-            // there is no later one
-            auto next = it;
-            ++next;
-            bool is_last = true;
-            for (; next != v.end(); ++next)
-            {
-                is_last = is_last && next.key() != it.key();
-            }
-            if (!is_last)
+            // lookups find the first member with the key, which is this one if
+            // there is no earlier one
+            if (std::find(keys.begin(), keys.end(), key) != keys.end())
             {
                 continue;
             }
+            keys.push_back(key);
             CHECK(v.find(key) == it);
             CHECK(v[key].materialize() == it->materialize());
             CHECK(v.at(key).materialize() == it.value().materialize());
@@ -725,20 +715,28 @@ TEST_CASE("json_view element access and iteration")
 #endif
     }
 
-    SECTION("duplicate keys: lookups find the last member, iteration all")
+    SECTION("duplicate keys: lookups find the first member, iteration all")
     {
         const json_document d = json_document::parse(R"({"a":1,"b":2,"a":3})");
         const json_view v = d.root();
         CHECK(v.size() == 3);
-        CHECK(v["a"].materialize() == 3);
-        CHECK(v.at("a").materialize() == 3);
-        CHECK(v.find("a") == std::next(v.begin(), 2));
-        CHECK(v.find("a").value().materialize() == 3);
+        CHECK(v["a"].materialize() == 1);
+        CHECK(v.at("a").materialize() == 1);
+        CHECK(v.find("a") == v.begin());
+        CHECK(v.find("a").value().materialize() == 1);
         CHECK(v.find("b") == std::next(v.begin()));
         CHECK(v.count("a") == 1);
         CHECK(v.contains("a"));
-        CHECK(v.value("a", 0) == 3);
-        CHECK(v["a"].materialize() == v.materialize()["a"]); // as materialize()
+        CHECK(v.value("a", 0) == 1);
+        CHECK(v.materialize()["a"] == 3); // materialize() keeps the last value, unlike the lookups
+        // JSON pointers resolve to the first member at every level
+        const json_document dp = json_document::parse(R"({"a":{"b":1},"a":{"b":2,"b":3}})");
+        CHECK(dp.root()[json::json_pointer("/a/b")].materialize() == 1);
+        CHECK(dp.root().at(json::json_pointer("/a/b")).materialize() == 1);
+        CHECK(dp.root().value(json::json_pointer("/a/b"), 0) == 1);
+        CHECK(dp.root().materialize()[json::json_pointer("/a/b")] == 3);
+        // get<map> keeps the last value, as materialize()
+        CHECK((v.get<std::map<std::string, int>>() == std::map<std::string, int> {{"a", 3}, {"b", 2}}));
         // keys of every length class (the 16-byte short compare and memcmp)
         for (const std::size_t n :
                 {
@@ -748,10 +746,10 @@ TEST_CASE("json_view element access and iteration")
             const std::string key(n, 'k');
             const json_document dk = json_document::parse("{\"" + key + "\":1,\"" + key + "x\":2,\"" + key + "\":3,\"" + key + "\":4}");
             CAPTURE(n)
-            CHECK(dk.root()[key].materialize() == 4);
-            CHECK(dk.root().at(key).materialize() == 4);
-            CHECK(dk.root().find(key) == std::next(dk.root().begin(), 3));
-            CHECK(dk.root().value(key, 0) == 4);
+            CHECK(dk.root()[key].materialize() == 1);
+            CHECK(dk.root().at(key).materialize() == 1);
+            CHECK(dk.root().find(key) == dk.root().begin());
+            CHECK(dk.root().value(key, 0) == 1);
             CHECK(dk.root()[key + "x"].materialize() == 2);
         }
         std::string order;
@@ -1515,6 +1513,8 @@ TEST_CASE("json_view dump")
         const json_document d = json_document::parse(R"({"b": 1, "a": 2, "b": 3})");
         CHECK(d.root().dump() == R"({"b":1,"a":2,"b":3})");
         CHECK(d.root().dump(1) == "{\n \"b\": 1,\n \"a\": 2,\n \"b\": 3\n}");
+        // dump() writes all members, though the lookup finds the first
+        CHECK(d.root()["b"].dump() == "1");
     }
 
     SECTION("deep nesting")
@@ -1637,6 +1637,9 @@ TEST_CASE("json_view comparison")
         const ordered_json_document dup = ordered_json_document::parse(R"({"a": 1, "b": 2, "a": 3})");
         const ordered_json_document last = ordered_json_document::parse(R"({"a": 3, "b": 2})");
         CHECK(dup.root() == last.root());
+        // (== compares as parse() resolves duplicates, a lookup finds the first)
+        CHECK(dup.root()["a"].materialize() == 1);
+        CHECK(dup.root()["a"] != last.root()["a"]);
         const ordered_json_document ab = ordered_json_document::parse(R"({"a": 1, "b": 2})");
         const ordered_json_document ba = ordered_json_document::parse(R"({"b": 2, "a": 1})");
         CHECK(ab.root() != ba.root());
@@ -1684,31 +1687,26 @@ TEST_CASE("json_view large objects")
         for (std::size_t i = 0; i < members; ++i)
         {
             const std::string key = std::string(i % 23, 'k') + std::to_string(i) + (i % 7 == 0 ? "\n" : "");
+            CHECK(v[key].get<std::size_t>() == i);
             CHECK(v.contains(key));
             CHECK(v.find(key).key() == key);
-            CHECK(!v.contains(key + "x"));
-            if (key == "k1")
-            {
-                continue; // repeated below: the last member wins
-            }
-            CHECK(v[key].get<std::size_t>() == i);
             CHECK(v.at(key).get<std::size_t>() == i);
+            CHECK(!v.contains(key + "x"));
         }
         CHECK(v[""].get_string() == "empty key");
-        CHECK(v["k1"].get_string() == "a duplicate of an earlier key"); // the last of duplicate keys, as for small objects
-        CHECK(v.at("k1").get_string() == "a duplicate of an earlier key");
+        CHECK(v["k1"].get<int>() == 1); // the first of duplicate keys, as for small objects
         CHECK(!v.contains("missing"));
         CHECK_THROWS_WITH_AS(v.at("missing"), "[json.exception.out_of_range.403] key 'missing' not found", json::out_of_range&);
         CHECK(v == j);
         CHECK(v.materialize() == j);
     }
 
-    SECTION("duplicate keys: the last member wins, with and without a table")
+    SECTION("duplicate keys: the first member wins, with and without a table")
     {
         // an object of `total` members: the keys "k0".."k<n-1>" in order, then
         // three keys repeated twice more (one copy in the middle, one at the
         // end), and two keys repeated once; the value of a member is its
-        // position, so that the last member of a key can be told apart
+        // position, so that the first member of a key can be told apart
         struct member
         {
             std::string key;
@@ -1756,20 +1754,23 @@ TEST_CASE("json_view large objects")
             REQUIRE(members.size() >= total);
             REQUIRE(members.size() >= 100);
             std::string text = "{";
+            std::map<std::string, std::size_t> first;
             std::map<std::string, std::size_t> last;
             for (const member& m : members)
             {
                 text += (text.size() > 1 ? ",\"" : "\"") + m.key + "\":" + std::to_string(m.position);
+                first.insert({m.key, m.position});
                 last[m.key] = m.position;
             }
             text += '}';
-            REQUIRE(last.size() < members.size());
+            REQUIRE(first.size() < members.size());
 
             const json_document d = json_document::parse(text);
             const json_view v = d.root();
             const json j = json::parse(text);
             CHECK(v.size() == members.size()); // every occurrence is visited
-            for (const auto& entry : last)
+            const json m = v.materialize();
+            for (const auto& entry : first)
             {
                 CAPTURE(entry.first)
                 const std::size_t expected = entry.second;
@@ -1784,7 +1785,9 @@ TEST_CASE("json_view large objects")
                 CHECK(v.at(json::json_pointer(pointer)).get<std::size_t>() == expected);
                 CHECK(v.value(json::json_pointer(pointer), std::size_t{0}) == expected);
                 CHECK(v.contains(json::json_pointer(pointer)));
-                CHECK(j[entry.first].get<std::size_t>() == expected); // as materialize() and parse() keep it
+                // materialize() and parse() keep the last value instead
+                CHECK(j[entry.first].get<std::size_t>() == last.at(entry.first));
+                CHECK(m[entry.first].get<std::size_t>() == last.at(entry.first));
             }
             CHECK(!v.contains("k"));
             CHECK(v["missing"].is_discarded());

tests/src/unit-json_view_edit.cpp

@@ -28,6 +28,7 @@ using ptr_t = ordered_json::json_pointer;
 #include <limits>
 #include <map>
 #include <random>
+#include <set>
 #include <string>
 #include <vector>
 
@@ -181,8 +182,10 @@ void compare(const ordered_json_editable_view& v, const ordered_json& j)
     }
 }
 
-// the text of j, in which some objects repeat a key of theirs: before their
-// members (the real member is then the last), or after (the repeat is)
+// the text of j, in which some objects repeat a key of theirs after their
+// members: lookups of the edited document find the first member of a key, so
+// j (the first members) is what they must agree with, while parse() would
+// keep the last value
 std::string text_with_duplicates(const ordered_json& j)
 {
     if (j.is_object())
@@ -197,10 +200,6 @@ std::string text_with_duplicates(const ordered_json& j)
             return ordered_json(keys[static_cast<std::size_t>(r(static_cast<int>(keys.size())))]).dump() + ":" + random_value(2).dump();
         };
         std::string text = "{";
-        if (!keys.empty() && r(4) == 0)
-        {
-            text += repeated() + ",";
-        }
         bool first = true;
         for (const auto& kv : j.items())
         {
@@ -225,10 +224,58 @@ std::string text_with_duplicates(const ordered_json& j)
     return j.dump();
 }
 
-// every lookup of the edited view finds what j holds, although the view may
-// have several members for a key (j has the last value, at the position of the
-// first member: what parse() and materialize() make of it)
-void check_lookups(const ordered_json_editable_view& v, const ordered_json& j)
+// every lookup of the edited view finds the first member of its key, which
+// the iteration of the view shows
+void check_lookups(const ordered_json_editable_view& v)
+{
+    if (v.is_object())
+    {
+        std::set<std::string> seen;
+        for (const auto item : v.items())
+        {
+            const std::string key(item.key().data(), item.key().size());
+            if (!seen.insert(key).second)
+            {
+                continue; // (not the first member of the key)
+            }
+            const ptr_t ptr = ptr_t() / key;
+            CAPTURE(key)
+            const ordered_json expected = item.value().materialize();
+            const ordered_json_editable_view m = v[key];
+            REQUIRE(!m.is_discarded());
+            CHECK(m.materialize() == expected);
+            CHECK(v.at(key).materialize() == expected);
+            CHECK(v[ptr].materialize() == expected);
+            CHECK(v.at(ptr).materialize() == expected);
+            CHECK(v.contains(key));
+            CHECK(v.contains(ptr));
+            CHECK(v.count(key) == 1);
+            const auto it = v.find(key);
+            REQUIRE(it != v.end());
+            CHECK((*it).materialize() == expected);
+            CHECK(v.value(ptr, ordered_json(nullptr)) == expected);
+            if (expected.is_string())
+            {
+                CHECK(v.value(key, std::string("-")) == expected.get<std::string>());
+            }
+            check_lookups(m);
+        }
+        CHECK(v["missing#key"].is_discarded());
+        CHECK(!v.contains("missing#key"));
+        CHECK(v.find("missing#key") == v.end());
+    }
+    else if (v.is_array())
+    {
+        for (std::size_t i = 0; i < v.size(); ++i)
+        {
+            check_lookups(v[i]);
+            check_lookups(v.at(i));
+        }
+    }
+}
+
+// every lookup of the edited view finds what the first members of j hold
+void check_first_members(const ordered_json_editable_view& v, const ordered_json& j)
 {
     REQUIRE(v.type() == j.type());
     if (j.is_object())
@@ -238,45 +285,39 @@ void check_lookups(const ordered_json_editable_view& v, const ordered_json& j)
             const std::string& key = kv.key();
             const ptr_t ptr = ptr_t() / key;
             CAPTURE(key)
-            const ordered_json_editable_view m = v[key];
-            REQUIRE(!m.is_discarded());
-            CHECK(m.materialize() == kv.value());
-            CHECK(v.at(key).materialize() == kv.value());
-            CHECK(v[ptr].materialize() == kv.value());
-            CHECK(v.at(ptr).materialize() == kv.value());
-            CHECK(v.contains(key));
+            REQUIRE(v.contains(key));
             CHECK(v.contains(ptr));
             CHECK(v.count(key) == 1);
-            const auto it = v.find(key);
-            REQUIRE(it != v.end());
-            CHECK((*it).materialize() == kv.value());
-            CHECK(v.value(ptr, ordered_json(nullptr)) == kv.value());
-            if (kv.value().is_string())
-            {
-                CHECK(v.value(key, std::string("-")) == kv.value().get<std::string>());
-            }
-            check_lookups(m, kv.value());
+            CHECK(v.find(key) != v.end());
+            CHECK(v.at(ptr).type() == kv.value().type());
+            check_first_members(v[key], kv.value());
+            check_first_members(v.at(key), kv.value());
+            check_first_members(v[ptr], kv.value());
         }
-        CHECK(v["missing#key"].is_discarded());
         CHECK(!v.contains("missing#key"));
-        CHECK(v.find("missing#key") == v.end());
     }
     else if (j.is_array())
     {
+        REQUIRE(v.size() == j.size());
         for (std::size_t i = 0; i < j.size(); ++i)
         {
-            check_lookups(v[i], j[i]);
-            check_lookups(v.at(i), j[i]);
+            check_first_members(v[i], j[i]);
         }
     }
+    else
+    {
+        CHECK(v.materialize() == j);
+    }
 }
 
-// (for documents whose objects repeat keys: dump(), size(), and iteration
-// list all members, so only the lookups and materialize() are compared)
+// (for documents whose objects repeat keys: j holds the first members, the
+// lookups must find them; dump(), size(), and iteration list all members, but
+// materialize() keeps the last value of a key, as parse() does of the dump)
 void check_duplicates(const ordered_json_editable_document& d, const ordered_json& j)
 {
-    CHECK(d.root().materialize() == j);
-    check_lookups(d.root(), j);
+    check_first_members(d.root(), j);
+    check_lookups(d.root());
+    CHECK(d.root().materialize() == ordered_json::parse(d.root().dump()));
 }
 
 void check_all(const ordered_json_editable_document& d, const ordered_json& j, bool deep)
@@ -304,17 +345,25 @@ TEST_CASE("json_view edits: differential")
     // materialize, and read back the same
     for (int n = 0; n < 250; ++n)
     {
-        // the last 100 documents repeat keys in some of their objects
+        // the last 100 documents repeat keys in some of their objects; j then
+        // holds the first member of each key, which the edits address
         const bool duplicates = n >= 150;
         ordered_json j = random_value(0);
         if (r(4) == 0)
         {
             j = ordered_json::object({{"a", random_value(1)}, {"b", random_value(1)}});
         }
+        if (duplicates)
+        {
+            j = ordered_json::parse(j.dump()); // (normalized as the document reads it)
+        }
         const std::string text = duplicates ? text_with_duplicates(j) : j.dump(r(2) == 0 ? -1 : 2);
         CAPTURE(text)
         ordered_json_editable_document d = ordered_json_editable_document::parse(text);
-        j = ordered_json::parse(text);
+        if (!duplicates)
+        {
+            j = ordered_json::parse(text);
+        }
         const ordered_json_document other = ordered_json_document::parse(random_value(0).dump());
         const int edits = 30;
         for (int e = 0; e < edits; ++e)
@@ -624,7 +673,7 @@ TEST_CASE("json_view edits: views and values")
     SECTION("duplicate keys")
     {
         json_editable_document d = json_editable_document::parse(R"({"a": 1, "b": 2, "a": 3})");
-        d.set(d.root(), "a", 4); // the last member is assigned (at the position of the first), the others dropped
+        d.set(d.root(), "a", 4); // the first member is assigned, the others dropped
         CHECK(d.root().dump() == R"({"a":4,"b":2})");
         d = json_editable_document::parse(R"({"a": 1, "b": 2, "a": 3})");
         CHECK(d.erase(d.root(), "a") == 2);
@@ -911,10 +960,10 @@ TEST_CASE("json_view edits: replacing arrays and objects by scalars")
 
 namespace
 {
-// reads of "a" in an object that repeats it: the last member wins, however the
+// reads of "a" in an object that repeats it: the first member wins, however the
 // object is stored
 template<typename View>
-void check_last_wins(const View& o, int expected)
+void check_first_wins(const View& o, int expected)
 {
     CAPTURE(expected)
     CHECK(o["a"].template get<int>() == expected);
@@ -935,35 +984,36 @@ TEST_CASE("json_view edits: duplicate keys")
     SECTION("an object in its parsed layout, then moved by edits")
     {
         json_editable_document d = json_editable_document::parse(R"({"a": 1, "b": 2, "a": 3})");
-        check_last_wins(d.root(), 3);
+        check_first_wins(d.root(), 1);
         CHECK(d.root().size() == 3);
         CHECK(d.root().dump() == R"({"a":1,"b":2,"a":3})");
+        CHECK(d.root().materialize()["a"] == 3); // (parse() keeps the last value)
 
         // an edit of a value does not move the object
         d.set(d.root()["b"], 5);
-        check_last_wins(d.root(), 3);
-        d.set(d.root()["a"], 4); // the member that reads find
-        check_last_wins(d.root(), 4);
-        CHECK(d.root().dump() == R"({"a":1,"b":5,"a":4})");
+        check_first_wins(d.root(), 1);
+        d.set(d.root()["a"], 4); // the member that lookups find
+        check_first_wins(d.root(), 4);
+        CHECK(d.root().dump() == R"({"a":4,"b":5,"a":3})");
 
         // an appended member moves it
         d.set(d.root(), "c", true);
-        check_last_wins(d.root(), 4);
+        check_first_wins(d.root(), 4);
         CHECK(d.root().size() == 4);
-        CHECK(d.root().dump() == R"({"a":1,"b":5,"a":4,"c":true})");
+        CHECK(d.root().dump() == R"({"a":4,"b":5,"a":3,"c":true})");
         d.set(d.root()["a"], 6);
-        check_last_wins(d.root(), 6);
-        CHECK(d.root().dump() == R"({"a":1,"b":5,"a":6,"c":true})");
+        check_first_wins(d.root(), 6);
+        CHECK(d.root().dump() == R"({"a":6,"b":5,"a":3,"c":true})");
         d.set(json::json_pointer("/a"), 7);
-        check_last_wins(d.root(), 7);
+        check_first_wins(d.root(), 7);
     }
 
-    SECTION("set assigns the member that reads find")
+    SECTION("set assigns the member that lookups find")
     {
-        // the key keeps the position of its first occurrence (as in materialize()), the later members are dropped
+        // the key stays where its first occurrence is, the later members are dropped
         ordered_json_editable_document d = ordered_json_editable_document::parse(R"({"a": 1, "b": 2, "a": 3, "c": 4, "a": 5})");
         const ordered_json_editable_view held = d.root()["a"];
-        CHECK(held.get<int>() == 5);
+        CHECK(held.get<int>() == 1);
         const ordered_json_editable_view assigned = d.set(d.root(), "a", "x");
         CHECK(held.get<std::string>() == "x");
         CHECK(assigned.get<std::string>() == "x");
@@ -979,7 +1029,7 @@ TEST_CASE("json_view edits: duplicate keys")
         // through a pointer, below a duplicate
         d = ordered_json_editable_document::parse(R"({"a": {"x": 1}, "b": 2, "a": {"x": 3}})");
         d.set(ptr_t("/a/x"), 4);
-        CHECK(d.root().dump() == R"({"a":{"x":1},"b":2,"a":{"x":4}})");
+        CHECK(d.root().dump() == R"({"a":{"x":4},"b":2,"a":{"x":3}})");
         d.set(ptr_t("/a"), 0);
         CHECK(d.root().dump() == R"({"a":0,"b":2})");
     }
@@ -988,7 +1038,7 @@ TEST_CASE("json_view edits: duplicate keys")
     {
         json_editable_document d = json_editable_document::parse(R"({"a": 1, "b": 2, "a": 3})");
         d.set(d.root(), "c", 4);
-        check_last_wins(d.root(), 3);
+        check_first_wins(d.root(), 1);
         CHECK(d.erase(d.root(), "a") == 2);
         CHECK(d.root().dump() == R"({"b":2,"c":4})");
         CHECK(!d.root().contains("a"));
@@ -1008,20 +1058,20 @@ TEST_CASE("json_view edits: duplicate keys")
         }
         text += R"(,"a":1,"k7":"last","a":2})";
         json_editable_document d = json_editable_document::parse(text);
-        check_last_wins(d.root(), 2);
-        CHECK(d.root()["k7"].get_string() == "last");
+        check_first_wins(d.root(), 0);
+        CHECK(d.root()["k7"].get_string() == "first");
         CHECK(d.root()["k199"].get<int>() == 199);
 
         // a value assigned in place: the index stays in use
         d.set(d.root()["a"], 3);
-        check_last_wins(d.root(), 3);
+        check_first_wins(d.root(), 3);
         d.set(d.root()["k7"], "changed");
         CHECK(d.root()["k7"].get_string() == "changed");
         CHECK(d.root()["k199"].get<int>() == 199);
 
         // an appended member moves the members: the lookup scans them
         d.set(d.root(), "new", 1);
-        check_last_wins(d.root(), 3);
+        check_first_wins(d.root(), 3);
         CHECK(d.root()["k7"].get_string() == "changed");
         CHECK(d.root()["k7"].get_string() == d.root().at("k7").get_string());
         CHECK(d.root()["k199"].get<int>() == 199);
@@ -1031,7 +1081,7 @@ TEST_CASE("json_view edits: duplicate keys")
         d = json_editable_document::parse(text);
         d.set(d.root()["a"], json{{"x", 1}});
         CHECK(d.root()["a"]["x"].get<int>() == 1);
-        CHECK(d.root()["k7"].get_string() == "last");
+        CHECK(d.root()["k7"].get_string() == "first");
         CHECK(d.erase(d.root(), "k7") == 3); // (the key occurs three times)
         CHECK(d.root()["k7"].is_discarded());
         CHECK(d.root()["a"]["x"].get<int>() == 1);
@@ -1040,18 +1090,18 @@ TEST_CASE("json_view edits: duplicate keys")
     SECTION("objects in moved arrays and in new values")
     {
         json_editable_document d = json_editable_document::parse(R"([{"a": 1, "a": 2}, {"a": 3, "b": 0, "a": 4}])");
-        check_last_wins(d.root()[0], 2);
-        check_last_wins(d.root()[1], 4);
+        check_first_wins(d.root()[0], 1);
+        check_first_wins(d.root()[1], 3);
         d.insert(d.root(), 0, json::parse(R"({"a": 0})"));
         d.push_back(d.root(), json::parse(R"({"a": 5, "a": 6})")); // (a basic_json value has one member)
-        check_last_wins(d.root()[1], 2);
-        check_last_wins(d.root()[2], 4);
+        check_first_wins(d.root()[1], 1);
+        check_first_wins(d.root()[2], 3);
         CHECK(d.root()[3]["a"].get<int>() == 6);
         d.set(d.root()[1], "a", 8);
-        check_last_wins(d.root()[1], 8);
+        check_first_wins(d.root()[1], 8);
         CHECK(d.root()[1].dump() == R"({"a":8})");
         d.erase(d.root(), 0);
-        check_last_wins(d.root()[1], 4);
+        check_first_wins(d.root()[1], 3);
         CHECK(d.root().dump() == R"([{"a":8},{"a":3,"b":0,"a":4},{"a":6}])");
     }
 
@@ -1063,14 +1113,14 @@ TEST_CASE("json_view edits: duplicate keys")
         d.set(d.root(), "o", source.root()["o"]);
         // (the copies keep the repeated members)
         CHECK(d.root().dump() == R"({"copy":[{"a":1,"a":2}],"o":{"b":{"a":3,"a":4},"a":5,"a":6}})");
-        check_last_wins(d.root()["copy"][0], 2);
-        check_last_wins(d.root()["o"]["b"], 4);
-        check_last_wins(d.root()["o"], 6);
+        check_first_wins(d.root()["copy"][0], 1);
+        check_first_wins(d.root()["o"]["b"], 3);
+        check_first_wins(d.root()["o"], 5);
         d.set(d.root()["o"]["b"], "c", 0);
-        check_last_wins(d.root()["o"]["b"], 4);
+        check_first_wins(d.root()["o"]["b"], 3);
         d.set(d.root()["o"], "d", 0);
-        check_last_wins(d.root()["o"], 6);
-        CHECK(d.root()["o"]["b"]["a"].get<int>() == 4);
+        check_first_wins(d.root()["o"], 5);
+        CHECK(d.root()["o"]["b"]["a"].get<int>() == 3);
         CHECK(d.root()["o"]["d"].get<int>() == 0);
     }
 }

tests/src/unit-json_view_image.cpp

@@ -241,7 +241,7 @@ TEST_CASE("json_view images: round trips")
             {
                 CHECK(l.root()["k" + std::to_string(i)] == d.root()["k" + std::to_string(i)]);
             }
-            CHECK(l.root()["k7"] == "a duplicate"); // the last of duplicate keys
+            CHECK(l.root()["k7"].get<int>() == 7); // the first of duplicate keys
             CHECK(l.root()["inner"]["m199"].get<int>() == -199);
             CHECK(!l.root().contains("k1000"));
             // the index is not part of the image
@@ -251,18 +251,18 @@ TEST_CASE("json_view images: round trips")
         CHECK(node_at(image, 0).extra == 0);
     }
 
-    SECTION("duplicate keys of a large object: lookups return the last member")
+    SECTION("duplicate keys of a large object: lookups return the first member")
     {
         std::string text = "{";
         for (int i = 0; i < 200; ++i)
         {
             text += (i != 0 ? ",\"k" : "\"k") + std::to_string(i) + "\":" + std::to_string(i);
         }
-        // three members for k5 (the third is the last), and duplicates of k0 and k199
+        // three members for k5 (the first is a number), and duplicates of k0 and k199
         text += R"(,"k5":"two","k0":null,"k199":[],"k5":"three"})";
         const json_document d = json_document::parse(text);
         REQUIRE(d.root().size() == 204);
-        CHECK(d.root()["k5"] == "three");
+        CHECK(d.root()["k5"] == 5);
         const std::vector<std::uint8_t> image = d.save();
         for (const image_check check :
                 {
@@ -271,13 +271,15 @@ TEST_CASE("json_view images: round trips")
         {
             const json_document l = json_document::load(image, check);
             CHECK(l.root().size() == 204);
-            CHECK(l.root()["k5"] == "three");
-            CHECK(l.root().at("k5") == "three");
-            CHECK(l.root().find("k5").value() == "three");
-            CHECK(l.root()["k0"].is_null());
-            CHECK(l.root()["k199"] == json::array());
+            CHECK(l.root()["k5"] == 5);
+            CHECK(l.root().at("k5") == 5);
+            CHECK(l.root().find("k5").value() == 5);
+            CHECK(l.root()["k0"] == 0);
+            CHECK(l.root()["k199"] == 199);
             CHECK(l.root()["k100"] == 100);
+            // materialize() keeps the semantics of parse(): the last value
             CHECK(l.root().materialize() == d.root().materialize());
+            CHECK(l.root().materialize()["k5"] == "three");
         }
     }
 
@@ -355,15 +357,8 @@ TEST_CASE("json_view images: round trips")
                 for (std::size_t i = 0; i < members; ++i)
                 {
                     CAPTURE(i)
-                    const bool duplicate = std::find(duplicated.begin(), duplicated.end(), keys[i]) != duplicated.end();
-                    if (duplicate)
-                    {
-                        CHECK(l.root()[keys[i]] == "last");
-                    }
-                    else
-                    {
-                        CHECK(l.root()[keys[i]] == i);
-                    }
+                    // lookups find the first member, also of the duplicated keys
+                    CHECK(l.root()[keys[i]] == i);
                     CHECK(l.root().at(keys[i]) == l.root()[keys[i]]);
                     CHECK(l.root().find(keys[i]).key() == keys[i]);
                     CHECK(!l.root().contains(keys[i] + "x"));