Niels Lohmann
481a9a216f
Add json_document and json_view, a read-only, zero-copy index of a JSON text, as the first public slice of the zero-copy view (#5295). A parse produces a flat array of 16-byte nodes in document order, one per value and one per object key. Strings stay in the source text; escaped strings are decoded into an arena. Integers are converted while their digits are in the cache; floats keep only their digit layout and are converted on read. Containers store the size of their subtree, so a reader can step over one in constant time. A document makes a handful of allocations, however many values it has. The parser accepts exactly what json::parse accepts, with every combination of ignore_comments and ignore_trailing_commas, with and without a trailing NUL, and under JSON_STRICT_NUL_HANDLING. It is portable C++11 and does not depend on byte order. basic_json_document adds parse, parse_copy, accept, read (reuses a document's memory), root, is_discarded, source, owns_source, node_count, memory_usage, and shrink_to_fit. basic_json_view adds type, the is_* queries, operator bool, size, empty, materialize, and source_offset. A parse error throws the same exception basic_json::parse would throw for the same input, message and position included. detail::abi_config keeps JSON_STRICT_NUL_HANDLING and JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON readable after json.hpp undefines them, in the ABI namespace so they always match the basic_json in use. A NUL byte that ends a // comment is the end of the input, as in parse() since #5696. Signed-off-by: Niels Lohmann <mail@nlohmann.me>
73 lines
1.9 KiB
Markdown
73 lines
1.9 KiB
Markdown
# <small>nlohmann::basic_json::</small>empty
|
|
|
|
```cpp
|
|
bool empty() const noexcept;
|
|
```
|
|
|
|
Checks if a JSON value has no elements (i.e., whether its [`size()`](size.md) 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`](array_t.md) and [`object_t`](object_t.md) satisfy the
|
|
[Container](https://en.cppreference.com/w/cpp/named_req/Container) concept; that is, their `empty()` functions have
|
|
constant complexity.
|
|
|
|
## Possible implementation
|
|
|
|
```cpp
|
|
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](size.md) returns the number of elements
|
|
- [clear](clear.md) clears the content and resets the value to the default value
|
|
- [basic_json_view::empty](../basic_json_view/empty.md) - 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.
|