string_view support for object access

cmake/ci.cmake

@@ -493,6 +493,7 @@ add_custom_target(ci_test_coverage
     COMMAND CXX=g++ ${CMAKE_COMMAND}
         -DCMAKE_BUILD_TYPE=Debug -GNinja -DCMAKE_CXX_FLAGS="--coverage;-fprofile-arcs;-ftest-coverage"
         -DJSON_BuildTests=ON -DJSON_MultipleHeaders=ON
+        -DCMAKE_CXX_STANDARD=17 -DCMAKE_CXX_STANDARD_REQUIRED=ON
         -S${PROJECT_SOURCE_DIR} -B${PROJECT_BINARY_DIR}/build_coverage
     COMMAND ${CMAKE_COMMAND} --build ${PROJECT_BINARY_DIR}/build_coverage
     COMMAND cd ${PROJECT_BINARY_DIR}/build_coverage && ${CMAKE_CTEST_COMMAND} --parallel ${N} --output-on-failure

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

@@ -6,8 +6,10 @@ reference at(size_type idx);
 const_reference at(size_type idx) const;
 
 // (2)
-reference at(const typename object_t::key_type& key);
-const_reference at(const typename object_t::key_type& key) const;
+template<typename KeyT>
+reference at(const KeyT& key);
+template<typename KeyT>
+const_reference at(const KeyT& key) const;
 
 // (3)
 reference at(const json_pointer& ptr);
@@ -18,6 +20,12 @@ const_reference at(const json_pointer& ptr) const;
 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.
 
+## Template parameters
+
+`KeyT`
+:   A type for an object key other than `basic_json::json_pointer` that is less-than comparable with `string_t`. This
+    can also be a string literal or a string view (C++17).
+
 ## Parameters
 
 `idx` (in)
@@ -172,5 +180,5 @@ Strong exception safety: if an exception occurs, the original value stays intact
 ## Version history
 
 1. Added in version 1.0.0.
-2. Added in version 1.0.0.
+2. Added in version 1.0.0; `KeyT` template added in version 3.10.5.
 3. Added in version 2.0.0.

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

@@ -3,7 +3,7 @@
 ```cpp
 // (1)
 template<typename KeyT>
-bool contains(KeyT && key) const;
+bool contains(const KeyT& key) const;
 
 // (2)
 bool contains(const json_pointer& ptr) const;
@@ -16,7 +16,8 @@ bool contains(const json_pointer& ptr) const;
 ## Template parameters
 
 `KeyT`
-:   A type for an object key other than `basic_json::json_pointer`.
+:   A type for an object key other than `basic_json::json_pointer` that is less-than comparable with `string_t`. This
+    can also be a string literal or a string view (C++17).
 
 ## Parameters
 
@@ -90,5 +91,5 @@ Logarithmic in the size of the JSON object.
 
 ## Version history
 
-1. Added in version 3.6.0.
+1. Added in version 3.6.0. Extended template `KeyT` in version 3.10.5 to also support `std::string_view`.
 2. Added in version 3.7.0.

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

@@ -2,7 +2,7 @@
 
 ```cpp
 template<typename KeyT>
-size_type count(KeyT&& key) const;
+size_type count(const KeyT& key) const;
 ```
 
 Returns the number of elements with key `key`. If `ObjectType` is the default `std::map` type, the return value will
@@ -11,7 +11,8 @@ always be `0` (`key` was not found) or `1` (`key` was found).
 ## Template parameters
 
 `KeyT`
-:   A type for an object key.
+:   A type for an object key that is less-than comparable with `string_t`. This can also be a string literal or a string
+    view (C++17).
 
 ## Parameters
 
@@ -53,3 +54,4 @@ This method always returns `0` when executed on a JSON type that is not an objec
 ## Version history
 
 - Added in version 1.0.0.
+- Extended template `KeyT` in version 3.10.5 to also support `std::string_view`.

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

@@ -10,7 +10,8 @@ iterator erase(iterator first, iterator last);
 const_iterator erase(const_iterator first, const_iterator last);
 
 // (3)
-size_type erase(const typename object_t::key_type& key);
+template<typename KeyT>
+size_type erase(const KeyT& key);
 
 // (4)
 void erase(const size_type idx);
@@ -31,6 +32,12 @@ void erase(const size_type idx);
 
 4. Removes an element from a JSON array by index.
 
+## Template parameters
+
+`KeyT`
+:   A type for an object key that is less-than comparable with `string_t`. This can also be a string literal or a string
+    view (C++17).
+
 ## Parameters
 
 `pos` (in)
@@ -174,3 +181,4 @@ Strong exception safety: if an exception occurs, the original value stays intact
 
 - Added in version 1.0.0.
 - Added support for binary types in version 3.8.0.
+- Added `KeyT` template in version 3.10.5.

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

@@ -2,10 +2,10 @@
 
 ```cpp
 template<typename KeyT>
-iterator find(KeyT&& key);
+iterator find(const KeyT& key);
 
 template<typename KeyT>
-const_iterator find(KeyT&& key) const;
+const_iterator find(const KeyT& 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
@@ -14,7 +14,8 @@ object, `end()` is returned.
 ## Template parameters
 
 `KeyT`
-:   A type for an object key.
+:   A type for an object key that is less-than comparable with `string_t`. This can also be a string literal or a string
+    view (C++17).
 
 ## Parameters
 
@@ -61,3 +62,4 @@ This method always returns `end()` when executed on a JSON type that is not an o
 ## Version history
 
 - Added in version 1.0.0.
+- Extended template `KeyT` in version 3.10.5 to also support `std::string_view`.

doc/mkdocs/docs/api/basic_json/operator[].md

@@ -6,12 +6,10 @@ reference operator[](size_type idx);
 const_reference operator[](size_type idx) const;
 
 // (2)
-reference operator[](const typename object_t::key_type& key);
-const_reference operator[](const typename object_t::key_type& key) const;
-template<typename T>
-reference operator[](T* key);
-template<typename T>
-const_reference operator[](T* key) const;
+template<typename KeyT>
+reference operator[](const KeyT& key);
+template<typename KeyT>
+const_reference operator[](const KeyT& key) const;
 
 // (3)
 reference operator[](const json_pointer& ptr);
@@ -24,8 +22,9 @@ const_reference operator[](const json_pointer& ptr) const;
 
 ## Template parameters
 
-`T`
-:   string literal convertible to `object_t::key_type`
+`KeyT`
+:   A type for an object key other than `basic_json::json_pointer` that is less-than comparable with `string_t`. This
+    can also be a string literal or a string view (C++17).
 
 ## Parameters
 
@@ -192,5 +191,6 @@ Strong exception safety: if an exception occurs, the original value stays intact
 ## Version history
 
 1. Added in version 1.0.0.
-2. Added in version 1.0.0. Overloads for `T* key` added in version 1.1.0.
+2. Added in version 1.0.0. Overloads for `T* key` added in version 1.1.0. Template `T* key` replaced by template `KeyT`
+   in version 3.10.5 which now also supports `std::string_view`.
 3. Added in version 2.0.0.

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

@@ -2,14 +2,14 @@
 
 ```cpp
 // (1)
-template<class ValueType>
-ValueType value(const typename object_t::key_type& key,
-                const ValueType& default_value) const;
+template<class KeyType, class ValueType>
+ValueType value(const KeyType& key,
+                ValueType && default_value) const;
 
 // (2)
 template<class ValueType>
 ValueType value(const json_pointer& ptr,
-                const ValueType& default_value) const;
+                ValueType && default_value) const;
 ```
 
 1. Returns either a copy of an object's element at the specified key `key` or a given default value if no element with
@@ -44,6 +44,9 @@ ValueType value(const json_pointer& ptr,
 
 ## Template parameters
 
+`KeyType`
+:   A type for an object key that is less-than comparable with `string_t`. This can also be a string literal or a string
+    view (C++17).
 `ValueType` 
 :   type compatible to JSON values, for instance `#!cpp int` for JSON integer numbers, `#!cpp bool` for JSON booleans,
     or `#!cpp std::vector` types for JSON arrays. Note the type of the expected value at `key`/`ptr` and the default
@@ -126,4 +129,4 @@ changes to any JSON value.
 ## Version history
 
 1. Added in version 1.0.0.
-2. Added in version 2.0.2.
+2. Added in version 2.0.2. `KeyType` template parameter added in 3.10.5.

include/nlohmann/detail/meta/type_traits.hpp

@@ -439,6 +439,33 @@ struct is_constructible_tuple : std::false_type {};
 template<typename T1, typename... Args>
 struct is_constructible_tuple<T1, std::tuple<Args...>> : conjunction<is_constructible<T1, Args>...> {};
 
+/// type to check if KeyType can be used as object key
+
+template<typename ComparatorType, typename KeyType, typename ObjectKeyType, typename = void>
+struct is_key_type_comparable
+{
+    static constexpr bool value = false;
+};
+
+template<typename ComparatorType, typename KeyType, typename ObjectKeyType>
+struct is_key_type_comparable<ComparatorType, KeyType, ObjectKeyType, void_t<
+decltype(ComparatorType()(std::declval<KeyType const&>(), std::declval<ObjectKeyType const&>())),
+decltype(ComparatorType()(std::declval<ObjectKeyType const&>(), std::declval<KeyType const&>()))
+        >>
+{
+    static constexpr bool value = true;
+};
+
+template<typename BasicJsonType, typename KeyType>
+struct is_usable_as_key_type
+{
+    static constexpr bool value =
+        is_key_type_comparable<typename BasicJsonType::object_comparator_t, typename BasicJsonType::object_t::key_type, KeyType>::value &&
+        !std::is_same<KeyType, typename BasicJsonType::iterator>::value &&
+        !std::is_same<KeyType, typename BasicJsonType::const_iterator>::value;
+};
+
+
 // a naive helper to check if a type is an ordered_map (exploits the fact that
 // ordered_map inherits capacity() from std::vector)
 template <typename T>