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67 lines
2.3 KiB
Markdown
67 lines
2.3 KiB
Markdown
# <small>nlohmann::basic_json::</small>json_base_class_t
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```cpp
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using json_base_class_t = detail::json_base_class<CustomBaseClass>;
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```
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The base class used to inject custom functionality into each instance of `basic_json`.
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Examples of such functionality might be metadata, additional member functions (e.g., visitors), or other application-specific code.
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## Template parameters
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`CustomBaseClass`
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: the base class to be added to `basic_json`
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## Notes
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### Default type
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The default value for `CustomBaseClass` is `void`. In this case, an
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[empty base class](https://en.cppreference.com/w/cpp/language/ebo) is used and no additional functionality is injected.
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### Limitations
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The type `CustomBaseClass` has to be a default-constructible, non-`final` class.
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`basic_json` only supports copy/move construction/assignment if `CustomBaseClass` does so as well.
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A `CustomBaseClass` with non-static data members forfeits `basic_json`'s
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[standard layout](https://en.cppreference.com/w/cpp/named_req/StandardLayoutType) guarantee. See
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[Template Parameter Requirements](../../features/types/template_parameters.md#custombaseclass).
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#### Name conflicts
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Since `basic_json` derives from `CustomBaseClass`, members of `basic_json` hide members of `CustomBaseClass` with the
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same name. Hidden members remain accessible via [`as_base_class`](as_base_class.md) or by casting the value to
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`json_base_class_t`.
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!!! warning "Avoid generic member names"
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Future versions of the library may add members to `basic_json` that hide members of `CustomBaseClass` that are
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accessible today. To reduce the risk of such conflicts, avoid generic names for the members of `CustomBaseClass`,
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for instance by using a distinctive prefix.
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## Examples
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??? example
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The following code shows how to inject custom data and methods for each node.
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```cpp
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--8<-- "examples/json_base_class_t.cpp"
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```
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Output:
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```json
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--8<-- "examples/json_base_class_t.output"
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```
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## See also
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- [as_base_class](as_base_class.md) - access the custom base class
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- [Template Parameter Requirements](../../features/types/template_parameters.md#custombaseclass) - the requirements for `CustomBaseClass`
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## Version history
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- Added in version 3.12.0.
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- Made a public member type in version 3.13.0; it was private before, so it could not be named outside the class.
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