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>

bool nlohmann::basic_json::empty ( ) const

inlinenoexcept

Checks if a JSON value has no elements.

Returns

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

Value type	return value
null	true
boolean	false
string	false
number	false
object	result of function object_t::empty()
array	result of function array_t::empty()

Complexity

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

Requirements

This function satisfies the Container requirements:

• The complexity is constant.
• Has the semantics of begin() == end().

Example

The following code uses empty to check if a json object contains any elements.

#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 empty()
    std::cout << std::boolalpha;
    std::cout << j_null.empty() << '\n';
    std::cout << j_boolean.empty() << '\n';
    std::cout << j_number_integer.empty() << '\n';
    std::cout << j_number_float.empty() << '\n';
    std::cout << j_object.empty() << '\n';
    std::cout << j_object_empty.empty() << '\n';
    std::cout << j_array.empty() << '\n';
    std::cout << j_array_empty.empty() << '\n';
    std::cout << j_string.empty() << '\n';
}

Output:

true
false
false
false
false
true
false
true
false

The example code above can be translated with

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

.