template<template< typename U, typename V, typename...Args > class ObjectType = std::map, template< typename U, typename...Args > class ArrayType = std::vector, class StringType = std::string, class BooleanType = bool, class NumberIntegerType = int64_t, class NumberFloatType = double, template< typename U > class AllocatorType = std::allocator>

size_type nlohmann::basic_json::size ( ) const

inlinenoexcept

Returns the number of elements in a JSON value.

Returns

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

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

Complexity

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

Requirements

This function satisfies the Container requirements:

• The complexity is constant.
• Has the semantics of std::distance(begin(), end()).

Example

The following code calls size on the different value types.

#include <json.hpp>

using namespace nlohmann;

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:

0
1
1
1
2
0
5
0
1

The example code above can be translated with

g++ -std=c++11 -Isrc doc/examples/size.cpp -o size 

.