nlohmann::basic_json_view::size

size_type size() const noexcept;

Returns the number of elements, as BasicJsonType::size() would for the same value.

Return value

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

Value type	return value
null	`0`
discarded	`0`
boolean	`1`
string	`1`
number	`1`
object	number of key/value pairs
array	number of elements

Exception safety

No-throw guarantee: this function never throws exceptions.

Complexity

Constant: for an object or array, the element count is stored in the index, not counted on demand.

Notes

As for BasicJsonType::size(), this does not return the length of a string value -- it is 1 for a string, regardless of its length.

If the source text has an object with a duplicate key, every occurrence counts towards its size() -- unlike materialize() (and BasicJsonType::parse()), which keeps only the last value for a repeated key. This means #!cpp v.size() can be larger than #!cpp v.materialize().size(). See the Notes on duplicate keys of operator[] for why lookups and iteration disagree on how many members there are.

Examples

??? example

The example below uses `size()` and [`empty()`](empty.md) 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.

2.9 KiB Raw Blame History