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nlohmann
256 lines
8.8 KiB
Markdown
256 lines
8.8 KiB
Markdown
# <small>nlohmann::basic_json::</small>operator[]
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```cpp
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// (1)
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reference operator[](size_type idx);
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const_reference operator[](size_type idx) const;
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// (2)
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reference operator[](typename object_t::key_type key);
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const_reference operator[](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 operator[](KeyType&& key);
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template<typename KeyType>
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const_reference operator[](KeyType&& key) const;
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// (4)
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reference operator[](const json_pointer& ptr);
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const_reference operator[](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`.
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2. Returns a reference to the object element with specified key `key`. The non-const qualified overload takes the key by
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value.
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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 with specified JSON pointer `ptr`.
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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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## Iterator invalidation
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For the non-const versions 1. and 4., when passing an **array** index that does not exist, it is created and filled with
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a `#!json null` value before a reference to it is returned. For this, a reallocation can happen, in which case all
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iterators (including the [`end()`](end.md) iterator) and all references to the elements are invalidated.
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For [`ordered_json`](../ordered_json.md), also passing an **object key** to the non-const versions 2., 3., and 4., a
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reallocation can happen which again invalidates all iterators and all references.
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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 element 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. (const) reference to the element at index `idx`
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2. (const) reference to the element at key `key`
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3. (const) reference to the element at key `key`
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4. (const) 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.305`](../../home/exceptions.md#jsonexceptiontype_error305) if the JSON value is not an array
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or null; in that case, using the `[]` operator with an index makes no sense.
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2. The function can throw the following exceptions:
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- Throws [`type_error.305`](../../home/exceptions.md#jsonexceptiontype_error305) if the JSON value is not an object
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or null; in that case, using the `[]` operator with a key makes no sense.
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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'.
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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.
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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` for the const version.
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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.
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- Throws [`out_of_range.410`](../../home/exceptions.md#jsonexceptionout_of_range410) if an array index in the passed
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JSON pointer `ptr` exceeds the range of `size_type` (e.g., on 32-bit platforms).
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## Complexity
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1. Constant if `idx` is in the range of the array. Otherwise, linear in `idx - size()`.
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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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## Notes
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!!! danger "Undefined behavior and runtime assertions"
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The following cases apply to the **const** overloads; the non-const overloads instead insert the missing element
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(see the notes below).
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1. If the element at index `idx` does not exist, the behavior is undefined.
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2. If the element with key `key` does not exist, the behavior is undefined and is **guarded by a
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[runtime assertion](../../features/assertions.md)**!
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1. The non-const version may add values: If `idx` is beyond the range of the array (i.e., `idx >= size()`), then the
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array is silently filled up with `#!json null` values to make `idx` a valid reference to the last stored element. In
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case the value was `#!json null` before, it is converted to an array.
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2. If `key` is not found in the object, then it is silently added to the object and filled with a `#!json null` value to
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make `key` a valid reference. In case the value was `#!json null` before, it is converted to an object.
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3. See 2.
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4. `null` values are created in arrays and objects if necessary.
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In particular:
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- If the JSON pointer points to an object key that does not exist, it is created and filled with a `#!json null`
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value before a reference to it is returned.
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- If the JSON pointer points to an array index that does not exist, it is created and filled with a `#!json null`
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value before a reference to it is returned. All indices between the current maximum and the given index are also
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filled with `#!json null`.
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- The special value `-` is treated as a synonym for the index past the end.
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## Examples
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??? example "Example: (1) access specified array element"
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The example below shows how array elements can be read and written using `[]` operator. Note the addition of
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`#!json null` values.
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```cpp
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--8<-- "examples/operator_array__size_type.cpp"
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```
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Output:
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```json
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--8<-- "examples/operator_array__size_type.output"
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```
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??? example "Example: (1) access specified array element (const)"
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The example below shows how array elements can be read using the `[]` operator.
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```cpp
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--8<-- "examples/operator_array__size_type_const.cpp"
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```
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Output:
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```json
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--8<-- "examples/operator_array__size_type_const.output"
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```
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??? example "Example: (2) access specified object element"
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The example below shows how object elements can be read and written using the `[]` operator.
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```cpp
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--8<-- "examples/operator_array__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/operator_array__object_t_key_type.output"
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```
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??? example "Example: (2) access specified object element (const)"
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The example below shows how object elements can be read using the `[]` operator.
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```cpp
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--8<-- "examples/operator_array__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/operator_array__object_t_key_type_const.output"
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```
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??? example "Example: (3) access specified object element using string_view"
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The example below shows how object elements can be read using the `[]` operator.
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```cpp
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--8<-- "examples/operator_array__keytype.c++17.cpp"
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```
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Output:
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```json
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--8<-- "examples/operator_array__keytype.c++17.output"
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```
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??? example "Example: (3) access specified object element using string_view (const)"
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The example below shows how object elements can be read using the `[]` operator.
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```cpp
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--8<-- "examples/operator_array__keytype_const.c++17.cpp"
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```
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Output:
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```json
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--8<-- "examples/operator_array__keytype_const.c++17.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 values can be read and written using JSON Pointers.
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```cpp
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--8<-- "examples/operator_array__json_pointer.cpp"
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```
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Output:
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```json
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--8<-- "examples/operator_array__json_pointer.output"
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```
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??? example "Example: (4) access specified element via JSON Pointer (const)"
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The example below shows how values can be read using JSON Pointers.
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```cpp
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--8<-- "examples/operator_array__json_pointer_const.cpp"
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```
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Output:
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```json
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--8<-- "examples/operator_array__json_pointer_const.output"
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```
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## See also
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- documentation on [unchecked access](../../features/element_access/unchecked_access.md)
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- documentation on [runtime assertions](../../features/assertions.md)
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- see [`at`](at.md) for access by reference with range checking
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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. Added overloads for `T* key` in version 1.1.0. Removed overloads for `T* key` (replaced by 3)
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in version 3.11.0.
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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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