Skip to content

# 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.

Back to top