Skip to content

basic_json::at

// (1)
reference at(size_type idx);
const_reference at(size_type idx) const;

// (2)
reference at(const typename object_t::key_type& key);
const_reference at(const typename object_t::key_type& key) const;

// (3)
reference at(const json_pointer& ptr);
const_reference at(const json_pointer& ptr) const;
  1. Returns a reference to the element at specified location idx, with bounds checking.
  2. Returns a reference to the element at with specified key key, with bounds checking.
  3. Returns a reference to the element at with specified JSON pointer ptr, with bounds checking.

Parameters

idx (in)
index of the element to access
key (in)
object key of the elements to remove
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

Exceptions

  1. The function can throw the following exceptions:
    • Throws type_error.304 if the JSON value is not an array; in this case, calling at with an index makes no sense. See example below.
    • Throws out_of_range.401 if the index idx is out of range of the array; that is, idx >= size(). See example below.
  2. The function can throw the following exceptions:
    • Throws type_error.304 if the JSON value is not an object; in this case, calling at with a key makes no sense. See example below.
    • Throws out_of_range.403 if the key key is is not stored in the object; that is, find(key) == end(). See example below.
  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'. See example below.
    • Throws parse_error.109 if an array index in the passed JSON pointer ptr is not a number. See example below.
    • Throws out_of_range.401 if an array index in the passed JSON pointer ptr is out of range. See example below.
    • Throws out_of_range.402 if the array index '-' is used in the passed JSON pointer ptr. As at provides checked access (and no elements are implicitly inserted), the index '-' is always invalid. See example below.
    • Throws out_of_range.403 if the JSON pointer describes a key of an object which cannot be found. See example below.
    • Throws out_of_range.404 if the JSON pointer ptr can not be resolved. See example below.

Exception safety

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

Complexity

  1. Constant
  2. Logarithmic in the size of the container.
  3. Constant

Example

Example

The example below shows how array elements can be read and written using at(). It also demonstrates the different exceptions that can be thrown.

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

using json = nlohmann::json;

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

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

    // change element at index 1 (second element) to "second"
    array.at(1) = "second";

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


    // exception type_error.304
    try
    {
        // use at() on a non-array type
        json str = "I am a string";
        str.at(0) = "Another string";
    }
    catch (json::type_error& e)
    {
        std::cout << e.what() << '\n';
    }

    // exception out_of_range.401
    try
    {
        // try to write beyond the array limit
        array.at(5) = "sixth";
    }
    catch (json::out_of_range& e)
    {
        std::cout << e.what() << '\n';
    }
}

Output:

"third"
["first","second","third","fourth"]
[json.exception.type_error.304] cannot use at() with string
[json.exception.out_of_range.401] array index 5 is out of range
Example

The example below shows how array elements can be read using at(). It also demonstrates the different exceptions that can be thrown.

#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';


    // exception type_error.304
    try
    {
        // use at() on a non-array type
        const json str = "I am a string";
        std::cout << str.at(0) << '\n';
    }
    catch (json::type_error& e)
    {
        std::cout << e.what() << '\n';
    }

    // exception out_of_range.401
    try
    {
        // try to read beyond the array limit
        std::cout << array.at(5) << '\n';
    }
    catch (json::out_of_range& e)
    {
        std::cout << e.what() << '\n';
    }
}

Output:

"third"
[json.exception.type_error.304] cannot use at() with string
[json.exception.out_of_range.401] array index 5 is out of range
Example

The example below shows how object elements can be read and written using at(). It also demonstrates the different exceptions that can be thrown.

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

using json = nlohmann::json;

int main()
{
    // create JSON object
    json object =
    {
        {"the good", "il buono"},
        {"the bad", "il cattivo"},
        {"the ugly", "il brutto"}
    };

    // output element with key "the ugly"
    std::cout << object.at("the ugly") << '\n';

    // change element with key "the bad"
    object.at("the bad") = "il cattivo";

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


    // exception type_error.304
    try
    {
        // use at() on a non-object type
        json str = "I am a string";
        str.at("the good") = "Another string";
    }
    catch (json::type_error& e)
    {
        std::cout << e.what() << '\n';
    }

    // exception out_of_range.401
    try
    {
        // try to write at a nonexisting key
        object.at("the fast") = "il rapido";
    }
    catch (json::out_of_range& e)
    {
        std::cout << e.what() << '\n';
    }
}

Output:

"il brutto"
{"the bad":"il cattivo","the good":"il buono","the ugly":"il brutto"}
[json.exception.type_error.304] cannot use at() with string
[json.exception.out_of_range.403] key 'the fast' not found
Example

The example below shows how object elements can be read using at(). It also demonstrates the different exceptions that can be thrown.

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

using json = nlohmann::json;

int main()
{
    // create JSON object
    const json object =
    {
        {"the good", "il buono"},
        {"the bad", "il cattivo"},
        {"the ugly", "il brutto"}
    };

    // output element with key "the ugly"
    std::cout << object.at("the ugly") << '\n';


    // exception type_error.304
    try
    {
        // use at() on a non-object type
        const json str = "I am a string";
        std::cout << str.at("the good") << '\n';
    }
    catch (json::type_error& e)
    {
        std::cout << e.what() << '\n';
    }

    // exception out_of_range.401
    try
    {
        // try to read from a nonexisting key
        std::cout << object.at("the fast") << '\n';
    }
    catch (json::out_of_range)
    {
        std::cout << "out of range" << '\n';
    }
}

Output:

"il brutto"
[json.exception.type_error.304] cannot use at() with string
out of range
Example

The example below shows how object elements can be read and written using at(). It also demonstrates the different exceptions that can be thrown.

#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.at("/number"_json_pointer) << '\n';
    // output element with JSON pointer "/string"
    std::cout << j.at("/string"_json_pointer) << '\n';
    // output element with JSON pointer "/array"
    std::cout << j.at("/array"_json_pointer) << '\n';
    // output element with JSON pointer "/array/1"
    std::cout << j.at("/array/1"_json_pointer) << '\n';

    // writing access

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

    // change an array element
    j.at("/array/1"_json_pointer) = 21;
    // output the changed array
    std::cout << j["array"] << '\n';


    // out_of_range.106
    try
    {
        // try to use an array index with leading '0'
        json::reference ref = j.at("/array/01"_json_pointer);
    }
    catch (json::parse_error& e)
    {
        std::cout << e.what() << '\n';
    }

    // out_of_range.109
    try
    {
        // try to use an array index that is not a number
        json::reference ref = j.at("/array/one"_json_pointer);
    }
    catch (json::parse_error& e)
    {
        std::cout << e.what() << '\n';
    }

    // out_of_range.401
    try
    {
        // try to use a an invalid array index
        json::reference ref = j.at("/array/4"_json_pointer);
    }
    catch (json::out_of_range& e)
    {
        std::cout << e.what() << '\n';
    }

    // out_of_range.402
    try
    {
        // try to use the array index '-'
        json::reference ref = j.at("/array/-"_json_pointer);
    }
    catch (json::out_of_range& e)
    {
        std::cout << e.what() << '\n';
    }

    // out_of_range.403
    try
    {
        // try to use a JSON pointer to an nonexistent object key
        json::const_reference ref = j.at("/foo"_json_pointer);
    }
    catch (json::out_of_range& e)
    {
        std::cout << e.what() << '\n';
    }

    // out_of_range.404
    try
    {
        // try to use a JSON pointer that cannot be resolved
        json::reference ref = j.at("/number/foo"_json_pointer);
    }
    catch (json::out_of_range& e)
    {
        std::cout << e.what() << '\n';
    }
}

Output:

1
"foo"
[1,2]
2
"bar"
[1,21]
[json.exception.parse_error.106] parse error: array index '01' must not begin with '0'
[json.exception.parse_error.109] parse error: array index 'one' is not a number
[json.exception.out_of_range.401] array index 4 is out of range
[json.exception.out_of_range.402] array index '-' (2) is out of range
[json.exception.out_of_range.403] key 'foo' not found
[json.exception.out_of_range.404] unresolved reference token 'foo'
Example

The example below shows how object elements can be read using at(). It also demonstrates the different exceptions that can be thrown.

#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.at("/number"_json_pointer) << '\n';
    // output element with JSON pointer "/string"
    std::cout << j.at("/string"_json_pointer) << '\n';
    // output element with JSON pointer "/array"
    std::cout << j.at("/array"_json_pointer) << '\n';
    // output element with JSON pointer "/array/1"
    std::cout << j.at("/array/1"_json_pointer) << '\n';

    // out_of_range.109
    try
    {
        // try to use an array index that is not a number
        json::const_reference ref = j.at("/array/one"_json_pointer);
    }
    catch (json::parse_error& e)
    {
        std::cout << e.what() << '\n';
    }

    // out_of_range.401
    try
    {
        // try to use a an invalid array index
        json::const_reference ref = j.at("/array/4"_json_pointer);
    }
    catch (json::out_of_range& e)
    {
        std::cout << e.what() << '\n';
    }

    // out_of_range.402
    try
    {
        // try to use the array index '-'
        json::const_reference ref = j.at("/array/-"_json_pointer);
    }
    catch (json::out_of_range& e)
    {
        std::cout << e.what() << '\n';
    }

    // out_of_range.403
    try
    {
        // try to use a JSON pointer to an nonexistent object key
        json::const_reference ref = j.at("/foo"_json_pointer);
    }
    catch (json::out_of_range& e)
    {
        std::cout << e.what() << '\n';
    }

    // out_of_range.404
    try
    {
        // try to use a JSON pointer that cannot be resolved
        json::const_reference ref = j.at("/number/foo"_json_pointer);
    }
    catch (json::out_of_range& e)
    {
        std::cout << e.what() << '\n';
    }
}

Output:

1
"foo"
[1,2]
2
[json.exception.parse_error.109] parse error: array index 'one' is not a number
[json.exception.out_of_range.401] array index 4 is out of range
[json.exception.out_of_range.402] array index '-' (2) is out of range
[json.exception.out_of_range.403] key 'foo' not found
[json.exception.out_of_range.404] unresolved reference token 'foo'

Version history

  1. Added in version 1.0.0.
  2. Added in version 1.0.0.
  3. Added in version 2.0.0.