Merge branch 'json-view/13-view-dump' into json-view/16-view-simd

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

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 +107,18 @@ 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.
+    [`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 +147,51 @@ 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. 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 +221,8 @@ 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

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
 inline const node* find_member(const document_data& d, const node* object, const char* key, std::size_t n) noexcept
 {
     if (NLOHMANN_VIEW_UNLIKELY(object->extra != 0))
@@ -95,7 +96,6 @@ inline const node* find_member(const document_data& d, const node* object, const
     }
     const node* const end = document_data::child_end(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);
@@ -103,19 +103,19 @@ inline const node* find_member(const document_data& d, const node* object, const
         {
             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 = document_data::first_child(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/serializer.hpp

@@ -46,11 +46,6 @@ 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

@@ -236,7 +236,7 @@ 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.
@@ -300,7 +300,7 @@ 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
@@ -358,7 +358,7 @@ 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
@@ -423,7 +423,7 @@ 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
     {
@@ -728,7 +728,7 @@ 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

@@ -3081,9 +3081,10 @@ 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
 inline const node* find_member(const document_data& d, const node* object, const char* key, std::size_t n) noexcept
 {
     if (NLOHMANN_VIEW_UNLIKELY(object->extra != 0))
@@ -3092,7 +3093,6 @@ inline const node* find_member(const document_data& d, const node* object, const
     }
     const node* const end = document_data::child_end(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);
@@ -3100,19 +3100,19 @@ inline const node* find_member(const document_data& d, const node* object, const
         {
             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 = document_data::first_child(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
@@ -3698,11 +3698,6 @@ 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)
@@ -4480,7 +4475,7 @@ 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.
@@ -4544,7 +4539,7 @@ 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
@@ -4602,7 +4597,7 @@ 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
@@ -4667,7 +4662,7 @@ 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
     {
@@ -4972,7 +4967,7 @@ 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;
@@ -1463,6 +1461,8 @@ 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")
@@ -1585,6 +1585,9 @@ 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());