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api/adl_serializer/from_json.md

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