Rework basic_json element access to accept more key types

Rework basic_json element access member functions and operators to
accept any type that meets the requirements defined by type trait
detail::is_usable_as_key_type.

Member functions and operators:
* at()
* operator[]
* value()
* erase()
* find()
* count()
* contains()

Update documentation to reflect these changes.

Add unit tests to excercise the new functions using std::string_view.

Co-authored-by: Niels Lohmann <niels.lohmann@gmail.com>

doc/mkdocs/docs/api/basic_json/at.md

@@ -10,13 +10,28 @@ reference at(const typename object_t::key_type& key);
 const_reference at(const typename object_t::key_type& key) const;
 
 // (3)
+template<typename KeyType>
+reference at(KeyType&& key);
+template<typename KeyType>
+const_reference at(KeyType&& key) const;
+
+// (4)
 reference at(const json_pointer& ptr);
 const_reference at(const json_pointer& ptr) const;
 ```
 
 1. Returns a reference to the array element at specified location `idx`, with bounds checking.
-2. Returns a reference to the object element at with specified key `key`, with bounds checking.
-3. Returns a reference to the element at with specified JSON pointer `ptr`, with bounds checking.
+2. Returns a reference to the object element with specified key `key`, with bounds checking.
+3. See 2. This overload is only available if `KeyType` is comparable with `#!cpp typename object_t::key_type` and
+   `#!cpp typename object_comparator_t::is_transparent` denotes a type.
+4. Returns a reference to the element at specified JSON pointer `ptr`, with bounds checking.
+
+## Template parameters
+
+`KeyType`
+:   A type for an object key other than [`json_pointer`](../json_pointer/index.md) that is comparable with
+    [`string_t`](string_t.md) using  [`object_comparator_t`](object_comparator_t.md).
+    This can also be a string view (C++17).
 
 ## Parameters
 
@@ -25,15 +40,16 @@ const_reference at(const json_pointer& ptr) const;
 
 `key` (in)
 :   object key of the elements to access
-    
+
 `ptr` (in)
 :   JSON pointer to the desired element
-    
+
 ## Return value
 
 1. reference to the element at index `idx`
 2. reference to the element at key `key`
-3. reference to the element pointed to by `ptr`
+3. reference to the element at key `key`
+4. reference to the element pointed to by `ptr`
 
 ## Exception safety
 
@@ -51,7 +67,8 @@ Strong exception safety: if an exception occurs, the original value stays intact
       in this case, calling `at` with a key makes no sense. See example below.
     - Throws [`out_of_range.403`](../../home/exceptions.md#jsonexceptionout_of_range403) if the key `key` is not
       stored in the object; that is, `find(key) == end()`. See example below.
-3. The function can throw the following exceptions:
+3. See 2.
+4. The function can throw the following exceptions:
     - Throws [`parse_error.106`](../../home/exceptions.md#jsonexceptionparse_error106) if an array index in the passed
       JSON pointer `ptr` begins with '0'. See example below.
     - Throws [`parse_error.109`](../../home/exceptions.md#jsonexceptionparse_error109) if an array index in the passed
@@ -68,9 +85,10 @@ Strong exception safety: if an exception occurs, the original value stays intact
 
 ## Complexity
 
-1. Constant
+1. Constant.
 2. Logarithmic in the size of the container.
-3. Constant
+3. Logarithmic in the size of the container.
+4. Logarithmic in the size of the container.
 
 ## Examples
 
@@ -134,7 +152,7 @@ Strong exception safety: if an exception occurs, the original value stays intact
     --8<-- "examples/at__object_t_key_type_const.output"
     ```
 
-??? example "Example (3) access specified element via JSON Pointer"
+??? example "Example (4) access specified element via JSON Pointer"
 
     The example below shows how object elements can be read and written using `at()`. It also demonstrates the different
     exceptions that can be thrown.
@@ -149,7 +167,7 @@ Strong exception safety: if an exception occurs, the original value stays intact
     --8<-- "examples/at_json_pointer.output"
     ```
 
-??? example "Example (3) access specified element via JSON Pointer"
+??? example "Example (4) access specified element via JSON Pointer"
 
     The example below shows how object elements can be read using `at()`. It also demonstrates the different exceptions
     that can be thrown.
@@ -173,4 +191,5 @@ Strong exception safety: if an exception occurs, the original value stays intact
 
 1. Added in version 1.0.0.
 2. Added in version 1.0.0.
-3. Added in version 2.0.0.
+3. Added in version 3.11.0.
+4. Added in version 2.0.0.

doc/mkdocs/docs/api/basic_json/basic_json.md

@@ -201,16 +201,16 @@ basic_json(basic_json&& other) noexcept;
 
 ## Exceptions
 
-1. /
+1. (none)
 2. The function does not throw exceptions.
-3. /
-4. /
+3. (none)
+4. (none)
 5. The function can throw the following exceptions:
     - Throws [`type_error.301`](../../home/exceptions.md#jsonexceptiontype_error301) if `type_deduction` is
       `#!cpp false`, `manual_type` is `value_t::object`, but `init` contains an element which is not a pair whose first
       element is a string. In this case, the constructor could not create an object. If `type_deduction` would have been
       `#!cpp true`, an array would have been created. See `object(initializer_list_t)` for an example.
-6. /
+6. (none)
 7. The function can throw the following exceptions:
     - Throws [`invalid_iterator.201`](../../home/exceptions.md#jsonexceptioninvalid_iterator201) if iterators `first`
       and `last` are not compatible (i.e., do not belong to the same JSON value). In this case, the range
@@ -220,7 +220,7 @@ basic_json(basic_json&& other) noexcept;
       element anymore. In this case, the range `[first, last)` is undefined. See example code below.
     - Throws [`invalid_iterator.206`](../../home/exceptions.md#jsonexceptioninvalid_iterator206) if iterators `first`
       and `last` belong to a `#!json null` value. In this case, the range `[first, last)` is undefined.
-8. /
+8. (none)
 9. The function does not throw exceptions.
 
 ## Complexity

doc/mkdocs/docs/api/basic_json/contains.md

@@ -2,21 +2,28 @@
 
 ```cpp
 // (1)
-template<typename KeyT>
-bool contains(KeyT && key) const;
+bool contains(const typename object_t::key_type& key) const;
 
 // (2)
+template<typename KeyType>
+bool contains(KeyType&& key) const;
+
+// (3)
 bool contains(const json_pointer& ptr) const;
 ```
 
-1. Check whether an element exists in a JSON object with key equivalent to `key`. If the element is not found or the 
+1. Check whether an element exists in a JSON object with a key equivalent to `key`. If the element is not found or the 
    JSON value is not an object, `#!cpp false` is returned.
-2. Check whether the given JSON pointer `ptr` can be resolved in the current JSON value.
+2. See 1. This overload is only available if `KeyType` is comparable with `#!cpp typename object_t::key_type` and
+   `#!cpp typename object_comparator_t::is_transparent` denotes a type.
+3. Check whether the given JSON pointer `ptr` can be resolved in the current JSON value.
 
 ## Template parameters
 
-`KeyT`
-:   A type for an object key other than `basic_json::json_pointer`.
+`KeyType`
+:   A type for an object key other than [`json_pointer`](../json_pointer/index.md) that is comparable with
+    [`string_t`](string_t.md) using  [`object_comparator_t`](object_comparator_t.md).
+    This can also be a string view (C++17).
 
 ## Parameters
 
@@ -30,7 +37,8 @@ bool contains(const json_pointer& ptr) const;
 
 1. `#!cpp true` if an element with specified `key` exists. If no such element with such key is found or the JSON value
    is not an object, `#!cpp false` is returned.
-2. `#!cpp true` if the JSON pointer can be resolved to a stored value, `#!cpp false` otherwise.
+2. See 1.
+3. `#!cpp true` if the JSON pointer can be resolved to a stored value, `#!cpp false` otherwise.
 
 ## Exception safety
 
@@ -39,7 +47,8 @@ Strong exception safety: if an exception occurs, the original value stays intact
 ## Exceptions
 
 1. The function does not throw exceptions.
-2. The function can throw the following exceptions:
+2. The function does not throw exceptions.
+3. The function can throw the following exceptions:
     - Throws [`parse_error.106`](../../home/exceptions.md#jsonexceptionparse_error106) if an array index begins with
       `0`.
     - Throws [`parse_error.109`](../../home/exceptions.md#jsonexceptionparse_error109) if an array index was not a
@@ -74,7 +83,7 @@ Logarithmic in the size of the JSON object.
     --8<-- "examples/contains.output"
     ```
 
-??? example "Example (1) check with JSON pointer"
+??? example "Example (3) check with JSON pointer"
 
     The example shows how `contains()` is used.
 
@@ -90,5 +99,6 @@ Logarithmic in the size of the JSON object.
 
 ## Version history
 
-1. Added in version 3.6.0.
-2. Added in version 3.7.0.
+1. Added in version 3.11.0.
+2. Added in version 3.6.0. Extended template `KeyType` to support comparable types in version 3.11.0.
+3. Added in version 3.7.0.

doc/mkdocs/docs/api/basic_json/count.md

@@ -1,17 +1,25 @@
 # <small>nlohmann::basic_json::</small>count
 
 ```cpp
-template<typename KeyT>
-size_type count(KeyT&& key) const;
+// (1)
+size_type count(const typename object_t::key_type& key) const;
+
+// (2)
+template<typename KeyType>
+size_type count(KeyType&& key) const;
 ```
 
-Returns the number of elements with key `key`. If `ObjectType` is the default `std::map` type, the return value will
-always be `0` (`key` was not found) or `1` (`key` was found).
+1. Returns the number of elements with key `key`. If `ObjectType` is the default `std::map` type, the return value will
+   always be `0` (`key` was not found) or `1` (`key` was found).
+2. See 1. This overload is only available if `KeyType` is comparable with `#!cpp typename object_t::key_type` and
+   `#!cpp typename object_comparator_t::is_transparent` denotes a type.
 
 ## Template parameters
 
-`KeyT`
-:   A type for an object key.
+`KeyType`
+:   A type for an object key other than [`json_pointer`](../json_pointer/index.md) that is comparable with
+    [`string_t`](string_t.md) using  [`object_comparator_t`](object_comparator_t.md).
+    This can also be a string view (C++17).
 
 ## Parameters
 
@@ -52,4 +60,5 @@ This method always returns `0` when executed on a JSON type that is not an objec
 
 ## Version history
 
-- Added in version 1.0.0.
+1. Added in version 3.11.0.
+2. Added in version 1.0.0. Changed parameter `key` type to `KeyType&&` in version 3.11.0.

doc/mkdocs/docs/api/basic_json/default_object_comparator_t.md

@@ -8,7 +8,8 @@ using default_object_comparator_t = std::less<>;            // since C++14
 
 The default comparator used by [`object_t`](object_t.md).
 
-Since C++14 a transparent comparator is used which prevents unnecessary string construction.
+Since C++14 a transparent comparator is used which prevents unnecessary string construction
+when looking up a key in an object.
 
 The actual comparator used depends on [`object_t`](object_t.md) and can be obtained via
 [`object_comparator_t`](object_comparator_t.md).

doc/mkdocs/docs/api/basic_json/erase.md

@@ -13,6 +13,10 @@ const_iterator erase(const_iterator first, const_iterator last);
 size_type erase(const typename object_t::key_type& key);
 
 // (4)
+template<typename KeyType>
+size_type erase(KeyType&& key);
+
+// (5)
 void erase(const size_type idx);
 ```
 
@@ -29,7 +33,17 @@ void erase(const size_type idx);
 
 3. Removes an element from a JSON object by key.
 
-4. Removes an element from a JSON array by index.
+4. See 3. This overload is only available if `KeyType` is comparable with `#!cpp typename object_t::key_type` and
+   `#!cpp typename object_comparator_t::is_transparent` denotes a type.
+
+5. Removes an element from a JSON array by index.
+
+## Template parameters
+
+`KeyType`
+:   A type for an object key other than [`json_pointer`](../json_pointer/index.md) that is comparable with
+    [`string_t`](string_t.md) using  [`object_comparator_t`](object_comparator_t.md).
+    This can also be a string view (C++17).
 
 ## Parameters
 
@@ -56,7 +70,8 @@ void erase(const size_type idx);
    is returned.
 3. Number of elements removed. If `ObjectType` is the default `std::map` type, the return value will always be `0`
    (`key` was not found) or `1` (`key` was found).
-4. /
+4. See 3.
+5. (none)
 
 ## Exception safety
 
@@ -83,7 +98,8 @@ Strong exception safety: if an exception occurs, the original value stays intact
 3. The function can throw the following exceptions:
     - Throws [`type_error.307`](../../home/exceptions.md#jsonexceptiontype_error307) when called on a type other than
       JSON object; example: `"cannot use erase() with null"`
-4. The function can throw the following exceptions:
+4. See 3.
+5. The function can throw the following exceptions:
     - Throws [`type_error.307`](../../home/exceptions.md#jsonexceptiontype_error307) when called on a type other than
       JSON object; example: `"cannot use erase() with null"`
     - Throws [`out_of_range.401`](../../home/exceptions.md#jsonexceptionout_of_range401) when `idx >= size()`; example:
@@ -103,14 +119,16 @@ Strong exception safety: if an exception occurs, the original value stays intact
        - strings and binary: linear in the length of the member
        - other types: constant
 3. `log(size()) + count(key)`
-4. Linear in distance between `idx` and the end of the container.
+4. `log(size()) + count(key)`
+5. Linear in distance between `idx` and the end of the container.
 
 ## Notes
 
 1. Invalidates iterators and references at or after the point of the `erase`, including the `end()` iterator.
-2. /
+2. (none)
 3. References and iterators to the erased elements are invalidated. Other references and iterators are not affected.
-4. /
+4. See 3.
+5. (none)
 
 ## Examples
 
@@ -156,7 +174,7 @@ Strong exception safety: if an exception occurs, the original value stays intact
     --8<-- "examples/erase__key_type.output"
     ```
 
-??? example "Example: (4) remove element from a JSON array given an index"
+??? example "Example: (5) remove element from a JSON array given an index"
 
     The example shows the effect of `erase()` using an array index.
 
@@ -172,5 +190,8 @@ Strong exception safety: if an exception occurs, the original value stays intact
 
 ## Version history
 
-- Added in version 1.0.0.
-- Added support for binary types in version 3.8.0.
+1. Added in version 1.0.0. Added support for binary types in version 3.8.0.
+2. Added in version 1.0.0. Added support for binary types in version 3.8.0.
+3. Added in version 1.0.0.
+4. Added in version 3.11.0.
+5. Added in version 1.0.0.

doc/mkdocs/docs/api/basic_json/find.md

@@ -1,20 +1,28 @@
 # <small>nlohmann::basic_json::</small>find
 
 ```cpp
-template<typename KeyT>
-iterator find(KeyT&& key);
-
-template<typename KeyT>
-const_iterator find(KeyT&& key) const;
+// (1)
+iterator find(const typename object_t::key_type& key);
+const_iterator find(const typename object_t::key_type& key) const;
+
+// (2)
+template<typename KeyType>
+iterator find(KeyType&& key);
+template<typename KeyType>
+const_iterator find(KeyType&& key) const;
 ```
 
-Finds an element in a JSON object with key equivalent to `key`. If the element is not found or the JSON value is not an
-object, `end()` is returned.
+1. Finds an element in a JSON object with a key equivalent to `key`. If the element is not found or the
+   JSON value is not an object, `end()` is returned.
+2. See 1. This overload is only available if `KeyType` is comparable with `#!cpp typename object_t::key_type` and
+   `#!cpp typename object_comparator_t::is_transparent` denotes a type.
 
 ## Template parameters
 
-`KeyT`
-:   A type for an object key.
+`KeyType`
+:   A type for an object key other than [`json_pointer`](../json_pointer/index.md) that is comparable with
+    [`string_t`](string_t.md) using  [`object_comparator_t`](object_comparator_t.md).
+    This can also be a string view (C++17).
 
 ## Parameters
 
@@ -23,8 +31,8 @@ object, `end()` is returned.
     
 ## Return value
 
-Iterator to an element with key equivalent to `key`. If no such element is found or the JSON value is not an object,
-past-the-end (see `end()`) iterator is returned.
+Iterator to an element with a key equivalent to `key`. If no such element is found or the JSON value is not an object,
+a past-the-end iterator (see `end()`) is returned.
 
 ## Exception safety
 
@@ -60,4 +68,5 @@ This method always returns `end()` when executed on a JSON type that is not an o
 
 ## Version history
 
-- Added in version 1.0.0.
+1. Added in version 3.11.0.
+2. Added in version 1.0.0. Changed to support comparable types in version 3.11.0.