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nlohmann
96 lines
2.4 KiB
Markdown
96 lines
2.4 KiB
Markdown
# JSON_BRACE_INIT_COPY_SEMANTICS
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```cpp
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#define JSON_BRACE_INIT_COPY_SEMANTICS /* value */
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```
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When defined to `1`, single-element brace initialization of a `basic_json` value is treated as a copy/move of the
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element rather than wrapping it in a single-element array.
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## Default definition
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The default value is `0` (disabled — existing behavior is preserved).
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```cpp
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#define JSON_BRACE_INIT_COPY_SEMANTICS 0
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```
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## Notes
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!!! note "Background"
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C++ always prefers the `initializer_list` constructor over the copy/move constructor for brace initialization. This
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means that code like
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```cpp
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json obj = {{"key", "value"}};
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json j{obj};
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```
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creates a single-element **array** `[{"key":"value"}]` instead of a copy of `obj`. This behavior is
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compiler-dependent for older compilers (GCC wrapped, Clang did not), but starting from Clang 20, both compilers
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behave the same way.
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Enabling this macro opts into copy/move semantics for this case
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(see [#5074](https://github.com/nlohmann/json/issues/5074)).
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!!! warning "Opt-in only"
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This macro must be defined **before** including `<nlohmann/json.hpp>`. Defining it after the include has no effect.
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!!! tip "Workaround without the macro"
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To explicitly create a single-element array without enabling this macro, use `json::array()`:
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```cpp
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json j = json::array({obj}); // always creates [obj]
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```
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## Examples
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??? example "Default behavior (macro not defined)"
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Without the macro, single-element brace initialization wraps the value in an array:
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```cpp
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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int main()
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{
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json obj = {{"key", "value"}};
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json j{obj};
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// j is [{"key":"value"}] -- single-element array, NOT a copy of obj
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}
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```
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??? example "Opt-in copy semantics (macro defined to 1)"
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With the macro, single-element brace initialization copies/moves the value:
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```cpp
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#define JSON_BRACE_INIT_COPY_SEMANTICS 1
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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int main()
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{
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json obj = {{"key", "value"}};
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json j{obj};
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// j is {"key":"value"} -- copy of obj
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}
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```
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## See also
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- [FAQ: Brace initialization yields arrays](../../home/faq.md#brace-initialization-yields-arrays)
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- [**basic_json(initializer_list_t)**](../basic_json/basic_json.md) - the affected constructor
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## Version history
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- Added in version 3.13.0.
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