nlohmann::basic_json::operator[]

// (1)
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;

// (3)
reference operator[](const json_pointer& ptr);
const_reference operator[](const json_pointer& ptr) const;

1. Returns a reference to the array element at specified location idx.
2. Returns a reference to the object element at with specified key key.
3. Returns a reference to the element at with specified JSON pointer ptr.

Template parameters

T : string literal convertible to object_t::key_type

Parameters

idx (in) : index of the element to access

key (in) : object key of the element 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

Exception safety

Strong exception safety: if an exception occurs, the original value stays intact.

Exceptions

1. The function can throw the following exceptions:
   • Throws type_error.305 if the JSON value is not an array or null; in that case, using the [] operator with an index makes no sense.
2. The function can throw the following exceptions:
   • Throws type_error.305 if the JSON value is not an object or null; in that case, using the [] operator with a key makes no sense.
3. The function can throw the following exceptions:
   • Throws parse_error.106 if an array index in the passed JSON pointer ptr begins with ‘0’.
   • Throws parse_error.109 if an array index in the passed JSON pointer ptr is not a number.
   • Throws out_of_range.402 if the array index ‘-’ is used in the passed JSON pointer ptr for the const version.
   • Throws out_of_range.404 if the JSON pointer ptr can not be resolved.

Complexity

1. Constant if idx is in the range of the array. Otherwise, linear in idx - size().
2. Logarithmic in the size of the container.
3. Constant

Notes

!!! danger

1. If the element with key `idx` does not exist, the behavior is undefined.
2. If the element with key `key` does not exist, the behavior is undefined and is **guarded by an assertion**!

1. The non-const version may add values: If idx is beyond the range of the array (i.e., idx >= size()), then the array is silently filled up with #!json null values to make idx a valid reference to the last stored element. In case the value was #!json null before, it is converted to an array.

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 make key a valid reference. In case the value was #!json null before, it is converted to an object.

3. null values are created in arrays and objects if necessary.

   In particular:

   • If the JSON pointer points to an object key that does not exist, it is created and filled with a #!json null value before a reference to it is returned.
   • If the JSON pointer points to an array index that does not exist, it is created and filled with a #!json null value before a reference to it is returned. All indices between the current maximum and the given index are also filled with #!json null.
   • The special value - is treated as a synonym for the index past the end.

Examples

??? example “Example (1): access specified array element”

The example below shows how array elements can be read and written using `[]` operator. Note the addition of
`#!json null` values.
    
```cpp
--8<-- "examples/operatorarray__size_type.cpp"
```

Output:

```json
--8<-- "examples/operatorarray__size_type.output"
```

??? example “Example (1): access specified array element”

The example below shows how array elements can be read using the `[]` operator.

```cpp
--8<-- "examples/operatorarray__size_type_const.cpp"
```

Output:

```json
--8<-- "examples/operatorarray__size_type_const.output"
```

??? example “Example (2): access specified object element”

The example below shows how object elements can be read and written using the `[]` operator.

```cpp
--8<-- "examples/operatorarray__key_type.cpp"
```

Output:

```json
--8<-- "examples/operatorarray__key_type.output"
```

??? example “Example (2): access specified object element”

The example below shows how object elements can be read using the `[]` operator.

```cpp
--8<-- "examples/operatorarray__key_type_const.cpp"
```

Output:

```json
--8<-- "examples/operatorarray__key_type_const.output"
```

??? example “Example (3): access specified element via JSON Pointer”

The example below shows how values can be read and written using JSON Pointers.

```cpp
--8<-- "examples/operatorjson_pointer.cpp"
```

Output:

```json
--8<-- "examples/operatorjson_pointer.output"
```

??? example “Example (3): access specified element via JSON Pointer”

The example below shows how values can be read using JSON Pointers.

```cpp
--8<-- "examples/operatorjson_pointer_const.cpp"
```

Output:

```json
--8<-- "examples/operatorjson_pointer_const.output"
```

See also

• see at for access by reference with range checking
• see value for access with default value

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.
3. Added in version 2.0.0.