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

nlohmann::basic_json::operator[]

// (1)
reference operator[](size_type idx);
const_reference operator[](size_type idx) const;

// (2)
reference operator[](typename object_t::key_type key);
const_reference operator[](const typename object_t::key_type& key) const;

// (3)
template<typename KeyType>
reference operator[](KeyType&& key);
template<typename KeyType>
const_reference operator[](KeyType&& key) const;

// (4)
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 with specified key key. The non-const qualified overload takes the key by value.
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 with specified JSON pointer ptr.

Template parameters

KeyType

A type for an object key other than json_pointer that is comparable with string_t using object_comparator_t. This can also be a string view (C++17).

Iterator invalidation

For the non-const versions 1. and 4., when passing an array index that does not exist, it is created and filled with a #!json null value before a reference to it is returned. For this, a reallocation can happen, in which case all iterators (including the end() iterator) and all references to the elements are invalidated.

For ordered_json, also passing an object key to the non-const versions 2., 3., and 4., a reallocation can happen which again invalidates all iterators and all references.

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. (const) reference to the element at index idx
2. (const) reference to the element at key key
3. (const) reference to the element at key key
4. (const) 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. See 2.
4. 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.
   • Throws out_of_range.410 if an array index in the passed JSON pointer ptr exceeds the range of size_type (e.g., on 32-bit platforms).

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. Logarithmic in the size of the container.
4. Logarithmic in the size of the container.

Notes

!!! danger "Undefined behavior and runtime assertions"

The following cases apply to the **const** overloads; the non-const overloads instead insert the missing element
(see the notes below).

1. If the element at index `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 a
   [runtime assertion](../../features/assertions.md)**!

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. See 2.

4. 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.

   !!! note "Creating intermediate levels that don't exist yet"

    When the JSON pointer traverses intermediate levels that don't exist at all yet (not just a missing
    leaf), each missing level is created as an array or an object depending on whether the corresponding
    pointer token parses as a non-negative integer: a numeric token creates an array, a non-numeric token
    creates an object. For example, on an initially `#!json null` value, `/foo/0/0/0` creates nested arrays,
    while `/foo/one/one/one` creates nested objects. This is not specified by the JSON Pointer RFC; it is
    this library's own, intentional disambiguation rule. See also [JSON Pointer](../../features/json_pointer.md).
   

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/operator_array__size_type.cpp"
```

Output:

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

??? example "Example: (1) access specified array element (const)"

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

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

Output:

```json
--8<-- "examples/operator_array__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/operator_array__object_t_key_type.cpp"
```

Output:

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

??? example "Example: (2) access specified object element (const)"

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

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

Output:

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

??? example "Example: (3) access specified object element using string_view"

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

```cpp
--8<-- "examples/operator_array__keytype.c++17.cpp"
```

Output:

```json
--8<-- "examples/operator_array__keytype.c++17.output"
```

??? example "Example: (3) access specified object element using string_view (const)"

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

```cpp
--8<-- "examples/operator_array__keytype_const.c++17.cpp"
```

Output:

```json
--8<-- "examples/operator_array__keytype_const.c++17.output"
```

??? example "Example: (4) access specified element via JSON Pointer"

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

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

Output:

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

??? example "Example: (4) access specified element via JSON Pointer (const)"

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

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

Output:

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

See also

• documentation on unchecked access
• documentation on runtime assertions
• 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. Added overloads for T* key in version 1.1.0. Removed overloads for T* key (replaced by 3) in version 3.11.0.
3. Added in version 3.11.0. Fixed in version 3.13.0 to consistently accept std::string_view-convertible keys, as already supported by at, value, find, and other lookup functions.
4. Added in version 2.0.0.