json/docs/mkdocs/docs/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
--8<-- "examples/empty.cpp"
```

Output:

```json
--8<-- "examples/empty.output"
```

See also

• size returns the number of elements
• clear clears the content and resets the value to the default value
• basic_json_view::empty - the same check on a zero-copy view

Version history

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