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# 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 `typename object_t::key_type` and `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).

## 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.

## 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

Undefined behavior and runtime assertions

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 a runtime 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 `null` values to make `idx` a valid reference to the last stored element. In case the value was `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 `null` value to make `key` a valid reference. In case the value was `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 `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 `null` value before a reference to it is returned. All indices between the current maximum and the given index are also filled with `null`.
   - The special value `-` is treated as a synonym for the index past the end.

## Examples

Example: (1) access specified array element

The example below shows how array elements can be read and written using `[]` operator. Note the addition of `null` values.

    #include <iostream>
    #include <nlohmann/json.hpp>

    using json = nlohmann::json;

    int main()
    {
        // create a JSON array
        json array = {1, 2, 3, 4, 5};

        // output element at index 3 (fourth element)
        std::cout << array[3] << '\n';

        // change last element to 6
        array[array.size() - 1] = 6;

        // output changed array
        std::cout << array << '\n';

        // write beyond array limit
        array[10] = 11;

        // output changed array
        std::cout << array << '\n';
    }

Output:

    4
    [1,2,3,4,6]
    [1,2,3,4,6,null,null,null,null,null,11]

Example: (1) access specified array element (const)

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

    #include <iostream>
    #include <nlohmann/json.hpp>

    using json = nlohmann::json;

    int main()
    {
        // create JSON array
        const json array = {"first", "2nd", "third", "fourth"};

        // output element at index 2 (third element)
        std::cout << array.at(2) << '\n';
    }

Output:

    "third"

Example: (2) access specified object element

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

    #include <iostream>
    #include <iomanip>
    #include <nlohmann/json.hpp>

    using json = nlohmann::json;

    int main()
    {
        // create a JSON object
        json object =
        {
            {"one", 1}, {"two", 2}, {"three", 2.9}
        };

        // output element with key "two"
        std::cout << object["two"] << "\n\n";

        // change element with key "three"
        object["three"] = 3;

        // output changed array
        std::cout << std::setw(4) << object << "\n\n";

        // mention nonexisting key
        object["four"];

        // write to nonexisting key
        object["five"]["really"]["nested"] = true;

        // output changed object
        std::cout << std::setw(4) << object << '\n';
    }

Output:

    2

    {
        "one": 1,
        "three": 3,
        "two": 2
    }

    {
        "five": {
            "really": {
                "nested": true
            }
        },
        "four": null,
        "one": 1,
        "three": 3,
        "two": 2
    }

Example: (2) access specified object element (const)

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

    #include <iostream>
    #include <nlohmann/json.hpp>

    using json = nlohmann::json;

    int main()
    {
        // create a JSON object
        const json object =
        {
            {"one", 1}, {"two", 2}, {"three", 2.9}
        };

        // output element with key "two"
        std::cout << object["two"] << '\n';
    }

Output:

    2

Example: (3) access specified object element using string_view

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

    #include <iostream>
    #include <iomanip>
    #include <string_view>
    #include <nlohmann/json.hpp>

    using namespace std::string_view_literals;
    using json = nlohmann::json;

    int main()
    {
        // create a JSON object
        json object =
        {
            {"one", 1}, {"two", 2}, {"three", 2.9}
        };

        // output element with key "two"
        std::cout << object["two"sv] << "\n\n";

        // change element with key "three"
        object["three"sv] = 3;

        // output changed array
        std::cout << std::setw(4) << object << "\n\n";

        // mention nonexisting key
        object["four"sv];

        // write to nonexisting key
        object["five"sv]["really"sv]["nested"sv] = true;

        // output changed object
        std::cout << std::setw(4) << object << '\n';
    }

Output:

    2

    {
        "one": 1,
        "three": 3,
        "two": 2
    }

    {
        "five": {
            "really": {
                "nested": true
            }
        },
        "four": null,
        "one": 1,
        "three": 3,
        "two": 2
    }

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

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

    #include <iostream>
    #include <string_view>
    #include <nlohmann/json.hpp>

    using namespace std::string_view_literals;
    using json = nlohmann::json;

    int main()
    {
        // create a JSON object
        const json object =
        {
            {"one", 1}, {"two", 2}, {"three", 2.9}
        };

        // output element with key "two"
        std::cout << object["two"sv] << '\n';
    }

Output:

    2

Example: (4) access specified element via JSON Pointer

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

    #include <iostream>
    #include <nlohmann/json.hpp>

    using json = nlohmann::json;
    using namespace nlohmann::literals;

    int main()
    {
        // create a JSON value
        json j =
        {
            {"number", 1}, {"string", "foo"}, {"array", {1, 2}}
        };

        // read-only access

        // output element with JSON pointer "/number"
        std::cout << j["/number"_json_pointer] << '\n';
        // output element with JSON pointer "/string"
        std::cout << j["/string"_json_pointer] << '\n';
        // output element with JSON pointer "/array"
        std::cout << j["/array"_json_pointer] << '\n';
        // output element with JSON pointer "/array/1"
        std::cout << j["/array/1"_json_pointer] << '\n';

        // writing access

        // change the string
        j["/string"_json_pointer] = "bar";
        // output the changed string
        std::cout << j["string"] << '\n';

        // "change" a nonexisting object entry
        j["/boolean"_json_pointer] = true;
        // output the changed object
        std::cout << j << '\n';

        // change an array element
        j["/array/1"_json_pointer] = 21;
        // "change" an array element with nonexisting index
        j["/array/4"_json_pointer] = 44;
        // output the changed array
        std::cout << j["array"] << '\n';

        // "change" the array element past the end
        j["/array/-"_json_pointer] = 55;
        // output the changed array
        std::cout << j["array"] << '\n';
    }

Output:

    1
    "foo"
    [1,2]
    2
    "bar"
    {"array":[1,2],"boolean":true,"number":1,"string":"bar"}
    [1,21,null,null,44]
    [1,21,null,null,44,55]

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

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

    #include <iostream>
    #include <nlohmann/json.hpp>

    using json = nlohmann::json;
    using namespace nlohmann::literals;

    int main()
    {
        // create a JSON value
        const json j =
        {
            {"number", 1}, {"string", "foo"}, {"array", {1, 2}}
        };

        // read-only access

        // output element with JSON pointer "/number"
        std::cout << j["/number"_json_pointer] << '\n';
        // output element with JSON pointer "/string"
        std::cout << j["/string"_json_pointer] << '\n';
        // output element with JSON pointer "/array"
        std::cout << j["/array"_json_pointer] << '\n';
        // output element with JSON pointer "/array/1"
        std::cout << j["/array/1"_json_pointer] << '\n';
    }

Output:

    1
    "foo"
    [1,2]
    2

## 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.
4. Added in version 2.0.0.

Last update: July 31, 2022
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