5352856f04
* Add key_compare member to ordered_map
* Replace == with key_compare in ordered_map
* Expose the actual comparison function used by object_t
nlohmann::ordered_map uses a different comparison function than the one
provided via template parameter.
* Introduce a type trait to detect if object_t has a key_compare member.
* Rename object_comparator_t to default_object_comparator_t.
* Add object_comparator_t to be conditionally defined as
object_t::key_compare, if available, or default_object_comparator_t
otherwise.
* Update the documentation accordingly.
Co-authored-by: Niels Lohmann <niels.lohmann@gmail.com>
* Add type traits to check if a type is usable as object key
Add type trait to check:
* if a type is a specialization of a template.
* if a type is a json_pointer.
* if a type is a basic_json::{const_,}iterator.
* if two types are comparable using a given comparison functor.
* if a type is comparable to basic_json::object_t::key_type.
* if a type has a member type is_transparent.
* if a type is usable as object key.
* if a type has an erase() function accepting a given KeyType.
Co-authored-by: Niels Lohmann <niels.lohmann@gmail.com>
* 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>
Co-authored-by: Niels Lohmann <niels.lohmann@gmail.com>
196 lines
6.5 KiB
Markdown
196 lines
6.5 KiB
Markdown
# <small>nlohmann::basic_json::</small>at
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```cpp
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// (1)
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reference at(size_type idx);
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const_reference at(size_type idx) const;
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// (2)
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reference at(const typename object_t::key_type& key);
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const_reference at(const typename object_t::key_type& key) const;
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// (3)
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template<typename KeyType>
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reference at(KeyType&& key);
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template<typename KeyType>
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const_reference at(KeyType&& key) const;
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// (4)
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reference at(const json_pointer& ptr);
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const_reference at(const json_pointer& ptr) const;
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```
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1. Returns a reference to the array element at specified location `idx`, with bounds checking.
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2. Returns a reference to the object element with specified key `key`, with bounds checking.
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3. See 2. This overload is only available if `KeyType` is comparable with `#!cpp typename object_t::key_type` and
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`#!cpp typename object_comparator_t::is_transparent` denotes a type.
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4. Returns a reference to the element at specified JSON pointer `ptr`, with bounds checking.
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## Template parameters
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`KeyType`
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: A type for an object key other than [`json_pointer`](../json_pointer/index.md) that is comparable with
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[`string_t`](string_t.md) using [`object_comparator_t`](object_comparator_t.md).
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This can also be a string view (C++17).
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## Parameters
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`idx` (in)
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: index of the element to access
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`key` (in)
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: object key of the elements to access
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`ptr` (in)
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: JSON pointer to the desired element
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## Return value
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1. reference to the element at index `idx`
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2. reference to the element at key `key`
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3. reference to the element at key `key`
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4. reference to the element pointed to by `ptr`
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## Exception safety
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Strong exception safety: if an exception occurs, the original value stays intact.
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## Exceptions
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1. The function can throw the following exceptions:
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- Throws [`type_error.304`](../../home/exceptions.md#jsonexceptiontype_error304) if the JSON value is not an array;
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in this case, calling `at` with an index makes no sense. See example below.
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- Throws [`out_of_range.401`](../../home/exceptions.md#jsonexceptionout_of_range401) if the index `idx` is out of
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range of the array; that is, `idx >= size()`. See example below.
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2. The function can throw the following exceptions:
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- Throws [`type_error.304`](../../home/exceptions.md#jsonexceptiontype_error304) if the JSON value is not an object;
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in this case, calling `at` with a key makes no sense. See example below.
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- Throws [`out_of_range.403`](../../home/exceptions.md#jsonexceptionout_of_range403) if the key `key` is not
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stored in the object; that is, `find(key) == end()`. See example below.
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3. See 2.
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4. The function can throw the following exceptions:
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- Throws [`parse_error.106`](../../home/exceptions.md#jsonexceptionparse_error106) if an array index in the passed
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JSON pointer `ptr` begins with '0'. See example below.
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- Throws [`parse_error.109`](../../home/exceptions.md#jsonexceptionparse_error109) if an array index in the passed
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JSON pointer `ptr` is not a number. See example below.
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- Throws [`out_of_range.401`](../../home/exceptions.md#jsonexceptionout_of_range401) if an array index in the passed
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JSON pointer `ptr` is out of range. See example below.
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- Throws [`out_of_range.402`](../../home/exceptions.md#jsonexceptionout_of_range402) if the array index '-' is used
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in the passed JSON pointer `ptr`. As `at` provides checked access (and no elements are implicitly inserted), the
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index '-' is always invalid. See example below.
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- Throws [`out_of_range.403`](../../home/exceptions.md#jsonexceptionout_of_range403) if the JSON pointer describes a
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key of an object which cannot be found. See example below.
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- Throws [`out_of_range.404`](../../home/exceptions.md#jsonexceptionout_of_range404) if the JSON pointer `ptr` can
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not be resolved. See example below.
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## Complexity
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1. Constant.
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2. Logarithmic in the size of the container.
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3. Logarithmic in the size of the container.
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4. Logarithmic in the size of the container.
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## Examples
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??? example "Example: (1) access specified array element with bounds checking"
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The example below shows how array elements can be read and written using `at()`. It also demonstrates the different
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exceptions that can be thrown.
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```cpp
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--8<-- "examples/at__size_type.cpp"
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```
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Output:
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```json
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--8<-- "examples/at__size_type.output"
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```
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??? example "Example: (1) access specified array element with bounds checking"
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The example below shows how array elements can be read using `at()`. It also demonstrates the different exceptions
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that can be thrown.
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```cpp
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--8<-- "examples/at__size_type_const.cpp"
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```
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Output:
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```json
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--8<-- "examples/at__size_type_const.output"
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```
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??? example "Example: (2) access specified object element with bounds checking"
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The example below shows how object elements can be read and written using `at()`. It also demonstrates the different
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exceptions that can be thrown.
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```cpp
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--8<-- "examples/at__object_t_key_type.cpp"
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```
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Output:
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```json
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--8<-- "examples/at__object_t_key_type.output"
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```
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??? example "Example (2) access specified object element with bounds checking"
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The example below shows how object elements can be read using `at()`. It also demonstrates the different exceptions
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that can be thrown.
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```cpp
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--8<-- "examples/at__object_t_key_type_const.cpp"
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```
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Output:
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```json
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--8<-- "examples/at__object_t_key_type_const.output"
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```
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??? example "Example (4) access specified element via JSON Pointer"
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The example below shows how object elements can be read and written using `at()`. It also demonstrates the different
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exceptions that can be thrown.
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```cpp
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--8<-- "examples/at_json_pointer.cpp"
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```
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Output:
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```json
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--8<-- "examples/at_json_pointer.output"
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```
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??? example "Example (4) access specified element via JSON Pointer"
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The example below shows how object elements can be read using `at()`. It also demonstrates the different exceptions
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that can be thrown.
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```cpp
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--8<-- "examples/at_json_pointer_const.cpp"
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```
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Output:
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```json
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--8<-- "examples/at_json_pointer_const.output"
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```
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## See also
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- see [`operator[]`](operator%5B%5D.md) for unchecked access by reference
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- see [`value`](value.md) for access with default value
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## Version history
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1. Added in version 1.0.0.
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2. Added in version 1.0.0.
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3. Added in version 3.11.0.
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4. Added in version 2.0.0.
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