NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE, NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT

#define NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(type, member...)              // (1)
#define NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT(type, member...) // (2)

These macros can be used to simplify the serialization/deserialization of types if you want to use a JSON object as serialization and want to use the member variable names as object keys in that object. The macro is to be defined outside the class/struct to create code for, but inside its namespace. Unlike NLOHMANN_DEFINE_TYPE_INTRUSIVE, it cannot access private members. The first parameter is the name of the class/struct, and all remaining parameters name the members.

Will use at during deserialization and will throw out_of_range.403 if a key is missing in the JSON object.
Will use value during deserialization and fall back to the default value for the respective type of the member variable if a key in the JSON object is missing. The generated from_json() function default constructs an object and uses its values as the defaults when calling the value function.

Parameters

type (in) : name of the type (class, struct) to serialize/deserialize

member (in) : name of the (public) member variable to serialize/deserialize; up to 64 members can be given as comma-separated list

Default definition

The macros add two functions to the namespace which take care of the serialization and deserialization:

void to_json(nlohmann::json&, const type&);
void from_json(const nlohmann::json&, type&);

See examples below for the concrete generated code.

Notes

!!! info “Prerequisites”

1. The type `type` must be default constructible. See [How can I use `get()` for non-default constructible/non-copyable types?][GetNonDefNonCopy]
   for how to overcome this limitation.
2. The macro must be used outside the type (class/struct).
3. The passed members must be public.

!!! warning “Implementation limits”

- The current implementation is limited to at most 64 member variables. If you want to serialize/deserialize types
  with more than 64 member variables, you need to define the `to_json`/`from_json` functions manually.
- The macros only work for the [`nlohmann::json`](../json.md) type; other specializations such as
  [`nlohmann::ordered_json`](../ordered_json.md) are currently unsupported.

Examples

??? example “Example (1): NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE”

Consider the following complete example:

```cpp hl_lines="15"
--8<-- "examples/nlohmann_define_type_non_intrusive_macro.cpp"
```

Output:

```json
--8<-- "examples/nlohmann_define_type_non_intrusive_macro.output"
```

Notes:

- `ns::person` is default-constructible. This is a requirement for using the macro.
- `ns::person` has only public member variables. This makes `NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE` applicable.
- The macro `NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE` is used _outside_ the class, but _inside_ its namespace `ns`.
- A missing key "age" in the deserialization yields an exception. To fall back to the default value,
  `NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT` can be used.

The macro is equivalent to:

```cpp hl_lines="15 16 17 18 19 20 21 22 23 24 25 26 27"
--8<-- "examples/nlohmann_define_type_non_intrusive_explicit.cpp"
```

??? example “Example (2): NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT”

Consider the following complete example:

```cpp hl_lines="20"
--8<-- "examples/nlohmann_define_type_non_intrusive_with_default_macro.cpp"
```

Output:

```json
--8<-- "examples/nlohmann_define_type_non_intrusive_with_default_macro.output"
```

Notes:

- `ns::person` is default-constructible. This is a requirement for using the macro.
- `ns::person` has only public member variables. This makes `NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT`
  applicable.
- The macro `NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT` is used _outside_ the class, but _inside_ its namespace `ns`.
- A missing key "age" in the deserialization does not yield an exception. Instead, the default value `-1` is used.

The macro is equivalent to:

```cpp hl_lines="20 21 22 23 24 25 26 27 28 29 30 31 32 33"
--8<-- "examples/nlohmann_define_type_non_intrusive_with_default_explicit.cpp"
```

Note how a default-initialized `person` object is used in the `from_json` to fill missing values.

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