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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>
1.9 KiB
1.9 KiB
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 falsefor binary types in version 3.8.0.