Make view lookups last-wins and chained access safe

* Lookups (operator[], at, find, contains, count, value, JSON pointers)
  return the last member of a duplicate key, as materialize() and
  parse() keep it.
* operator[] on a discarded view returns a discarded view instead of
  throwing, so v["a"]["b"] is safe for a missing "a".
* operator[] and at() take any integer type (not only int and size_t),
  fixing ambiguous calls with unsigned, long, std::int64_t, ...
* Fix the operator[] documentation, which claimed a discarded view for
  a type mismatch where type_error.305 is thrown.

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

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

@@ -8,15 +8,18 @@ basic_json_view at(const string_t& key) const;
 
 // (2)
 basic_json_view at(size_type idx) const;
-basic_json_view at(int idx) const;
+template<typename IntegerType>
+basic_json_view at(IntegerType idx) const;
 
 // (3)
 basic_json_view at(const json_pointer& ptr) const;
 ```
 
-1. Returns the value of the object member with key `key` -- the first one, should the key occur more than once (see
+1. Returns the value of the object member with key `key` -- the last one, should the key occur more than once (see
    [Notes on duplicate keys](operator[].md#notes)).
-2. Returns the array element at index `idx`.
+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
+   `idx` is out of range.
 3. Returns the value a JSON pointer `ptr` refers to, starting at this value.
 
 ## Parameters
@@ -32,7 +35,7 @@ basic_json_view at(const json_pointer& ptr) const;
 
 ## Return value
 
-1. the value of the first member with key `key`
+1. the value of the last member with key `key`
 2. the element at index `idx`
 3. the value `ptr` resolves to, starting at this value
 
@@ -72,8 +75,8 @@ 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, 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.
+   another, in document order, scanning all of them, since the last match is wanted. Each comparison first checks the
+   key's length -- already known from the index, without reading the key bytes -- before comparing its content.
 2. Linear in `idx`: elements are skipped one at a time from the first one, since they are not a fixed size in the
    index (unlike `BasicJsonType`'s array, which is random-access).
 3. Linear in the number of reference tokens of `ptr` and, for each token, in the number of members of the object at

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 *first* member with a given key. See the
+which all resolve to the *last* 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

@@ -33,8 +33,8 @@ 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, 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.
+   another, in document order, scanning all of them, since the last match is wanted. Each comparison first checks the
+   key's length -- already known from the index, without reading the key bytes -- before comparing its content.
 2. Linear in the number of reference tokens of `ptr` and, for each token, in the number of members of the object at
    that level or the index into the array -- as for [`operator[]`](operator[].md#complexity) and
    [`at`](at.md#complexity) with a JSON pointer.

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

@@ -23,9 +23,9 @@ 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, 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.
+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
+-- already known from the index, without reading the key bytes -- before comparing its content.
 
 ## Notes
 
@@ -36,7 +36,7 @@ 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, not just finding the first one.
+counting every member with a matching key (the lookup functions resolve to the *last* 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 first one, should the key occur more than once (see
+Finds a member with key `key` -- the last 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.
 
@@ -25,9 +25,9 @@ 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, 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.
+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
+-- already known from the index, without reading the key bytes -- before comparing its content.
 
 ## Notes
 

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 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)).
+    [`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)).
 
 !!! info "No pointers, references, or implicit conversion"
 

docs/mkdocs/docs/api/basic_json_view/is_discarded.md

@@ -9,6 +9,10 @@ view (see [(constructor)](basic_json_view.md)), and for [`root()`](../basic_json
 that is itself [discarded](../basic_json_document/is_discarded.md) -- in particular, the root of a failed
 [`parse()`](../basic_json_document/parse.md) with `allow_exceptions` set to `#!cpp false`.
 
+A discarded view is also what [`operator[]`](operator[].md) returns for a missing key, an index out of range, or a
+JSON pointer that cannot be resolved, and for any access on a view that is itself discarded (so a chain such as
+`#!cpp v["a"]["b"]` is safe). [`at`](at.md) throws instead.
+
 ## Return value
 
 `#!cpp true` if the view is discarded, `#!cpp false` otherwise.
@@ -45,6 +49,7 @@ site.
 
 ## See also
 
+- [operator[]](operator[].md) - access specified element; yields a discarded view where an element is missing
 - [operator bool](operator_bool.md) - return whether the view refers to a value
 - [(constructor)](basic_json_view.md) - the default constructor creates a discarded view
 - [is_discarded (basic_json_document)](../basic_json_document/is_discarded.md) - return whether the last parse failed

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 *first* member with a given key. See the
+[`contains`](contains.md), and [`count`](count.md), which resolve to the *last* 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) only ever sees the *first* one and [`materialize()`](materialize.md) -- like
-    [`BasicJsonType::parse()`](../basic_json/parse.md) -- keeps only the *last*.
+    [`operator[]`](operator[].md) sees the *last* one, and so does [`materialize()`](materialize.md) -- like
+    [`BasicJsonType::parse()`](../basic_json/parse.md).
 
     ```cpp
     --8<-- "examples/basic_json_view__items.cpp"

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

@@ -8,16 +8,19 @@ basic_json_view operator[](const string_t& key) const;
 
 // (2)
 basic_json_view operator[](size_type idx) const;
-basic_json_view operator[](int idx) const;
+template<typename IntegerType>
+basic_json_view operator[](IntegerType idx) const;
 
 // (3)
 basic_json_view operator[](const json_pointer& ptr) const;
 ```
 
-1. Returns the value of the object member with key `key` -- the first one, should the key occur more than once (see
+1. Returns the value of the object member with key `key` -- the last 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
-   `#!cpp int` overload only exists so that an integer literal is not ambiguous between this overload and 1.)
+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`,
+   `#!cpp long`, `#!cpp std::int64_t`, ...) and forwards to the `size_type` overload, so that an integer argument is
+   not ambiguous between that overload and 1; a negative `idx` is out of range.
 3. Returns the value a JSON pointer `ptr` refers to, starting at this value, or a [discarded](is_discarded.md) view
    wherever resolving it further is not possible without inserting into or extending the document (see
    [Return value](#return-value) and [Exceptions](#exceptions) below).
@@ -35,10 +38,12 @@ basic_json_view operator[](const json_pointer& ptr) const;
 
 ## Return value
 
-1. the value of the first member with key `key`, or a discarded view if `#!cpp is_object()` is `#!cpp false` or no
-   member has this key
-2. the element at index `idx`, or a discarded view if `#!cpp is_array()` is `#!cpp false` or `#!cpp idx >= size()`
-3. the value `ptr` resolves to, starting at this value, or a discarded view for exactly the reference tokens where the
+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
+   [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))
+3. the value `ptr` resolves to, starting at this value, or a discarded view (also if this view is
+   [discarded](is_discarded.md)) for exactly the reference tokens where the
    **const** overload of [`BasicJsonType::operator[]`](../basic_json/operator%5B%5D.md) invokes undefined behavior for
    the same pointer and the same document: an object member that does not exist, or an array index that is out of
    range
@@ -49,10 +54,12 @@ Strong exception safety: if an exception is thrown, there are no changes to the
 
 ## Exceptions
 
-1. Throws [`type_error.305`](../../home/exceptions.md#jsonexceptiontype_error305) if the value is not an object --
+1. Throws [`type_error.305`](../../home/exceptions.md#jsonexceptiontype_error305) if the value is not an object and
+   not [discarded](is_discarded.md) --
    the same exception, with the same message, that the **const** overload of
    [`BasicJsonType::operator[]`](../basic_json/operator%5B%5D.md) throws for a string argument on a non-object value.
-2. Throws [`type_error.305`](../../home/exceptions.md#jsonexceptiontype_error305) if the value is not an array --
+2. Throws [`type_error.305`](../../home/exceptions.md#jsonexceptiontype_error305) if the value is not an array and
+   not [discarded](is_discarded.md) --
    the same exception, with the same message, that the **const** overload of
    [`BasicJsonType::operator[]`](../basic_json/operator%5B%5D.md) throws for a numeric argument on a non-array value.
 3. Throws the same exceptions, with the same messages, that the **const** overload of
@@ -73,9 +80,9 @@ 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, 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.
+   another, in document order, scanning all of them, since the last match is wanted. 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.
 2. Linear in `idx`: elements are skipped one at a time from the first one, since they are not a fixed size in the
    index (unlike `BasicJsonType`'s array, which is random-access).
 3. Linear in the number of reference tokens of `ptr` and, for each token, in the number of members of the object at
@@ -84,20 +91,29 @@ None of these exceptions carry a [`JSON_DIAGNOSTICS`](../macros/json_diagnostics
 ## Notes
 
 Unlike `BasicJsonType::operator[]`, which is undefined behavior (guarded by a
-[runtime assertion](../../features/assertions.md)) for a missing key on a **const** value, this operator always
-returns a safe, testable result: a [discarded](is_discarded.md) view, which is `#!cpp false` in a boolean context.
+[runtime assertion](../../features/assertions.md)) for a missing key on a **const** value, this operator returns a
+safe, testable result for a missing key or an index out of range: a [discarded](is_discarded.md) view, which is
+`#!cpp false` in a boolean context.
 There is also no non-const overload that inserts a missing key or extends an array -- a view never modifies the
 document.
 
+!!! info "Chained access"
+
+    `#!cpp operator[]` on a [discarded](is_discarded.md) view returns a discarded view and does not throw, so a chain
+    like `#!cpp v["a"]["b"][0]` is safe even if `"a"` or `"b"` is missing: the first missing step makes the whole
+    result discarded, which is tested once at the end. Type errors on values that are *not* discarded still throw: a
+    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"
 
     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)) all resolve to 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 end up keeping the *last* value for a repeated key -- there is no reason for them to stop
-    early. [`begin()`](begin.md)/[`end()`](end.md) and [`items()`](items.md) iterate over *all* members, including
-    duplicates, in document order. See the example below and [`size()`](size.md#notes).
+    [`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 scans all members for this: it cannot stop at the first match. See the
+    example below and [`size()`](size.md#notes).
 
 !!! info "JSON pointer resolution"
 

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 first one, should the key occur more than once (see
+1. Returns the value of the object member with key `key` -- the last 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 first member with key `key`, converted to `T`, or `default_value`
+1. the last member with key `key`, converted to `T`, or `default_value`
 2. the value `ptr` resolves to, converted to `T`, or `default_value`
 
 ## Exception safety
@@ -68,8 +68,8 @@ 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, stopping at the first match. Plus the complexity of converting the found member to
-   `T` (see [`get`](get.md)).
+   another, in document order, scanning all of them, since the last match is wanted. Plus the complexity of converting
+   the found member to `T` (see [`get`](get.md)).
 2. Linear in the number of reference tokens of `ptr` and, for each token, in the number of members of the object at
    that level or the index into the array -- as for the [`operator[]`](operator[].md#complexity) and
    [`at`](at.md#complexity) overloads that take a JSON pointer. Plus the complexity of converting the resolved value

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[] only ever sees the first
-    // one, and materialize() -- like basic_json::parse() -- keeps the last
+    // the update history is visible; operator[] and materialize() -- like
+    // basic_json::parse() -- see the last one
     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 << "first \"retries\" seen by operator[]: " << settings["retries"].materialize().dump() << '\n';
+    std::cout << "last \"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
-first "retries" seen by operator[]: 1
+last "retries" seen by operator[]: 5
 last "retries" kept by materialize(): 5

docs/mkdocs/docs/features/json_view.md

@@ -126,14 +126,20 @@ whenever any of the other conditions above was not met.
   members in the order they appear in the source text. `basic_json`'s default `object_t` is a `std::map`, which
   sorts by key, so iterating a [`materialize()`](../api/basic_json_view/materialize.md)d value can print members in
   a different order than iterating the view they came from.
+- **Chained access is safe.** [`operator[]`](../api/basic_json_view/operator%5B%5D.md) with a missing key, an index
+  out of range, or an unresolvable JSON pointer returns a [discarded](../api/basic_json_view/is_discarded.md) view, and
+  `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
-  *first* occurrence, since a lookup can stop as soon as it finds a match. `basic_json::parse()` (and so
-  [`materialize()`](../api/basic_json_view/materialize.md)) instead keeps only the *last* value for a repeated key.
+  *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.
   See the [Notes on duplicate keys](../api/basic_json_view/operator%5B%5D.md#notes) of `operator[]`.
 - **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