nlohmann::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)
template<typename KeyType>
reference at(KeyType&& key);
template<typename KeyType>
const_reference at(KeyType&& key) const;

// (4)
reference at(const json_pointer& ptr);
const_reference at(const json_pointer& ptr) const;

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

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 elements 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 at key key
4. 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.304 if the JSON value is not an array; in this case, calling at with an index makes no sense. See the example below.
   • Throws out_of_range.401 if the index idx is out of range of the array; that is, idx >= size(). See the 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 the example below.
   • Throws out_of_range.403 if the key key is not stored in the object; that is, find(key) == end(). See the example below.
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'. See the example below.
   • Throws parse_error.109 if an array index in the passed JSON pointer ptr is not a number. See the example below.
   • Throws out_of_range.401 if an array index in the passed JSON pointer ptr is out of range. See the 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 the example below.
   • Throws out_of_range.403 if the JSON pointer describes a key of an object which cannot be found. See the example below.
   • Throws out_of_range.404 if the JSON pointer ptr can not be resolved. See the example below.

Complexity

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

Examples

Example: (1) access specified array element with bounds checking

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 (const 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 (const 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: (1) access specified array element with bounds checking

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 (const 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 (const 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: (2) access specified object element with bounds checking

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 (const 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 (const 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: (2) access specified object element with bounds checking

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 (const 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 (const 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: (3) access specified object element using string_view with bounds checking

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

using namespace std::string_view_literals;
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" using string_view
    std::cout << object.at("the ugly"sv) << '\n';

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

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

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

    // exception out_of_range.401
    try
    {
        // try to write at a nonexisting key using string_view
        object.at("the fast"sv) = "il rapido";
    }
    catch (const 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: (3) access specified object element using string_view with bounds checking

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

using namespace std::string_view_literals;
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" using string_view
    std::cout << object.at("the ugly"sv) << '\n';

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

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

Output:

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

Example: (4) access specified element via JSON Pointer

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;
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.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 (const 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 (const json::parse_error& e)
    {
        std::cout << e.what() << '\n';
    }

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

    // out_of_range.403
    try
    {
        // try to use a JSON pointer to a nonexistent object key
        json::const_reference ref = j.at("/foo"_json_pointer);
    }
    catch (const 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 (const 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: (4) access specified element via JSON Pointer

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;
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.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 (const json::parse_error& e)
    {
        std::cout << e.what() << '\n';
    }

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

    // out_of_range.403
    try
    {
        // try to use a JSON pointer to a nonexistent object key
        json::const_reference ref = j.at("/foo"_json_pointer);
    }
    catch (const 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 (const 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'

See also

• documentation on checked access
• operator[] for unchecked access by reference
• value for access with default value

Version history

1. Added in version 1.0.0.
2. Added in version 1.0.0.
3. Added in version 3.11.0.
4. Added in version 2.0.0.