nlohmann::basic_json::empty¶

bool empty() const noexcept;

Checks if a JSON value has no elements (i.e. whether its size() is 0).

Return value¶

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

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

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 empty() functions have constant complexity.

Possible implementation¶

bool empty() const noexcept
{
    return size() == 0;
}

Notes¶

This function does not return whether a string stored as JSON value is empty -- it returns whether the JSON container itself is empty which is false in the case of a string.

Examples¶

Example

The following code uses empty() to check if a JSON object contains any elements.

#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 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

Version history¶

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

Last update: May 1, 2022