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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

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 (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;

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;

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

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: May 17, 2022
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