json/api/basic_json/empty.md

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	#!cpp true
boolean	#!cpp false
string	#!cpp false
number	#!cpp false
binary	#!cpp 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 #!cpp false in the case of a string.

Examples

??? example

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

```cpp
#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:

```json
true
false
false
false
false
true
false
true
false

```

See also

• size returns the number of elements
• clear clears the content and resets the value to the default value

Version history

• Added in version 1.0.0.
• Extended to return #!cpp false for binary types in version 3.8.0.