nlohmann::basic_json::size¶

size_type size() const noexcept;

Returns the number of elements in a JSON value.

Return value¶

The return value depends on the different types and is defined as follows:

Value type	return value
null	`0`
boolean	`1`
string	`1`
number	`1`
binary	`1`
object	result of function object_t::size()
array	result of function array_t::size()

Exception safety¶

No-throw guarantee: this function never throws exceptions.

Complexity¶

Constant, as long as array_t and object_t satisfy the Container concept; that is, their size() functions have constant complexity.

Notes¶

This function does not return the length of a string stored as JSON value -- it returns the number of elements in the JSON value which is 1 in the case of a string.

Examples¶

Example

The following code calls size() on the different value types.

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

using json = nlohmann::json;

int main()
{
    // create JSON values
    json j_null;
    json j_boolean = true;
    json j_number_integer = 17;
    json j_number_float = 23.42;
    json j_object = {{"one", 1}, {"two", 2}};
    json j_object_empty(json::value_t::object);
    json j_array = {1, 2, 4, 8, 16};
    json j_array_empty(json::value_t::array);
    json j_string = "Hello, world";

    // call size()
    std::cout << j_null.size() << '\n';
    std::cout << j_boolean.size() << '\n';
    std::cout << j_number_integer.size() << '\n';
    std::cout << j_number_float.size() << '\n';
    std::cout << j_object.size() << '\n';
    std::cout << j_object_empty.size() << '\n';
    std::cout << j_array.size() << '\n';
    std::cout << j_array_empty.size() << '\n';
    std::cout << j_string.size() << '\n';
}

Output:

Version history¶

Added in version 1.0.0.
Extended to return 1 for binary types in version 3.8.0.

Last update: May 1, 2022