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nlohmann
75 lines
2.2 KiB
Markdown
75 lines
2.2 KiB
Markdown
# <small>nlohmann::adl_serializer::</small>from_json
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```cpp
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// (1)
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template<typename BasicJsonType, typename TargetType = ValueType>
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static auto from_json(BasicJsonType && j, TargetType& val) noexcept(
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noexcept(::nlohmann::from_json(std::forward<BasicJsonType>(j), val)))
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-> decltype(::nlohmann::from_json(std::forward<BasicJsonType>(j), val), void())
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// (2)
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template<typename BasicJsonType, typename TargetType = ValueType>
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static auto from_json(BasicJsonType && j) noexcept(
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noexcept(::nlohmann::from_json(std::forward<BasicJsonType>(j), detail::identity_tag<TargetType> {})))
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-> decltype(::nlohmann::from_json(std::forward<BasicJsonType>(j), detail::identity_tag<TargetType> {}))
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```
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This function is usually called by the [`get()`](../basic_json/get.md) function of the [basic_json](../basic_json/index.md)
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class (either explicitly or via the conversion operators).
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1. This function is chosen for default-constructible value types.
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2. This function is chosen for value types which are not default-constructible.
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## Parameters
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`j` (in)
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: JSON value to read from
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`val` (out)
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: value to write to
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## Return value
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1. (none) -- the converted value is written to the output parameter `val`.
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2. the JSON value `j` converted to `TargetType`
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## Examples
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??? example "Example: (1) Default-constructible type"
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The example below shows how a `from_json` function can be implemented for a user-defined type. This function is
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called by the `adl_serializer` when `get<ns::person>()` is called.
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```cpp
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--8<-- "examples/from_json__default_constructible.cpp"
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```
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Output:
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```json
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--8<-- "examples/from_json__default_constructible.output"
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```
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??? example "Example: (2) Non-default-constructible type"
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The example below shows how a `from_json` is implemented as part of a specialization of the `adl_serializer` to
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realize the conversion of a non-default-constructible type.
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```cpp
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--8<-- "examples/from_json__non_default_constructible.cpp"
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```
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Output:
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```json
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--8<-- "examples/from_json__non_default_constructible.output"
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```
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## See also
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- [to_json](to_json.md)
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## Version history
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- Added in version 2.1.0.
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