nlohmann::basic_json_view::empty

bool empty() const noexcept;

Checks whether size() is 0, as BasicJsonType::empty() would for the same value.

Return value

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

Value type	return value
null	`#!cpp true`
discarded	`#!cpp true`
boolean	`#!cpp false`
string	`#!cpp false`
number	`#!cpp false`
object	`#!cpp object_t::empty()`
array	`#!cpp array_t::empty()`

Exception safety

No-throw guarantee: this function never throws exceptions.

Complexity

Constant.

Notes

As for BasicJsonType::empty(), this does not return whether a string value is empty -- it is #!cpp false for any string, regardless of its length.

Examples

??? example

The example below uses [`size()`](size.md) and `empty()` to decide whether a parsed message is worth acting on,
without materializing it into a `BasicJsonType` value.

```cpp
#include <iostream>
#include <nlohmann/json_view.hpp>

using json_document = nlohmann::json_document;

int main()
{
    std::cout << std::boolalpha;

    // decide whether a batch is worth processing before building any
    // nlohmann::json value for it
    json_document batch = json_document::parse(R"([1, 2, 3, 4, 5])");
    json_document empty_batch = json_document::parse("[]");

    std::cout << batch.root().empty() << ' ' << batch.root().size() << '\n';
    std::cout << empty_batch.root().empty() << ' ' << empty_batch.root().size() << '\n';

    // as for basic_json: null has size 0, every other scalar has size 1
    json_document n = json_document::parse("null");
    json_document s = json_document::parse(R"("hi")");
    std::cout << n.root().size() << ' ' << s.root().size() << '\n';
}

```

Output:

```json
false 5
true 0
0 1

```

Version history

Added in version 3.13.0.

2.4 KiB Raw Blame History