json/docs/mkdocs/docs/api/adl_serializer/from_json.md

nlohmann::adl_serializer::from_json

// (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() function of the basic_json 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

Copy of the JSON value, converted to ValueType

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 `template 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

Version history

• Added in version 2.1.0.