87 lines
2.4 KiB
Markdown
87 lines
2.4 KiB
Markdown
# <small>nlohmann::basic_json_view::</small>empty
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```cpp
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bool empty() const noexcept;
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```
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Checks whether [`size()`](size.md) is `0`, as [`BasicJsonType::empty()`](../basic_json/empty.md) would for the same
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value.
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## Return value
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The return value depends on the type and is defined as follows:
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| Value type | return value |
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|----------------------|-----------------|
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| null | `#!cpp true` |
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| discarded | `#!cpp true` |
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| boolean | `#!cpp false` |
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| string | `#!cpp false` |
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| number | `#!cpp false` |
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| object | `#!cpp object_t::empty()` |
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| array | `#!cpp array_t::empty()` |
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## Exception safety
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No-throw guarantee: this function never throws exceptions.
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## Complexity
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Constant.
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## Notes
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As for [`BasicJsonType::empty()`](../basic_json/empty.md), this does not return whether a string value is empty -- it
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is `#!cpp false` for any string, regardless of its length.
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## Examples
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??? example
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The example below uses [`size()`](size.md) and `empty()` to decide whether a parsed message is worth acting on,
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without materializing it into a `BasicJsonType` value.
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```cpp
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#include <iostream>
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#include <nlohmann/json_view.hpp>
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using json_document = nlohmann::json_document;
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int main()
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{
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std::cout << std::boolalpha;
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// decide whether a batch is worth processing before building any
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// nlohmann::json value for it
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json_document batch = json_document::parse(R"([1, 2, 3, 4, 5])");
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json_document empty_batch = json_document::parse("[]");
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std::cout << batch.root().empty() << ' ' << batch.root().size() << '\n';
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std::cout << empty_batch.root().empty() << ' ' << empty_batch.root().size() << '\n';
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// as for basic_json: null has size 0, every other scalar has size 1
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json_document n = json_document::parse("null");
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json_document s = json_document::parse(R"("hi")");
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std::cout << n.root().size() << ' ' << s.root().size() << '\n';
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}
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```
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Output:
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```json
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false 5
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true 0
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0 1
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```
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## See also
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- [size](size.md) - return the number of elements
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- [`BasicJsonType::empty`](../basic_json/empty.md) - the corresponding function of `basic_json`
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## Version history
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- Added in version 3.13.0.
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