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docs/mkdocs/docs/api/macros/nlohmann_define_derived_type.md

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+# NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT
+
+# NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT
+
+```cpp
+#define NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE(type, base_type, member...)                  // (1)
+#define NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT(type, base_type, member...)     // (2)
+
+#define NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE(type, base_type, member...)              // (3)
+#define NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT(type, base_type, member...) // (4)
+```
+
+These macros can be used to simplify the serialization/deserialization of derived 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.
+
+- Macros 1 and 2 are to be defined **inside** the class/struct to create code for.
+Like [`NLOHMANN_DEFINE_TYPE_INTRUSIVE`](nlohmann_define_type_intrusive.md), they can access private members.
+- Macros 3 and 4 are to be defined **outside** the class/struct to create code for, but **inside** its namespace.
+Like [`NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE`](nlohmann_define_type_non_intrusive.md),
+they **cannot** access private members.
+
+
+The first  parameter is the name of the derived class/struct,
+the second parameter is the name of the base class/struct and all remaining parameters name the members.
+The base type **must** be already serializable/deserializable.
+
+- Macros 1 and 3 will use [`at`](../basic_json/at.md) during deserialization and will throw
+  [`out_of_range.403`](../../home/exceptions.md#jsonexceptionout_of_range403) if a key is missing in the JSON object.
+- Macros 2 and 4 will use [`value`](../basic_json/value.md) 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
+
+`base_type` (in)
+:   name of the base type (class, struct) `type` is derived from
+
+`member` (in)
+:   name of the member variable to serialize/deserialize; up to 64 members can be given as comma-separated list
+
+## Default definition
+
+Macros 1 and 2 add two friend functions to the class which take care of the serialization and deserialization:
+
+```cpp
+friend void to_json(nlohmann::json&, const type&);
+friend void from_json(const nlohmann::json&, type&);
+```
+
+Macros 3 and 4 add two functions to the namespace which take care of the serialization and deserialization:
+
+```cpp
+void to_json(nlohmann::json&, const type&);
+void from_json(const nlohmann::json&, type&);
+```
+
+In both cases they call the `to_json`/`from_json` functions of the base type
+before serializing/deserializing the members of the derived type:
+
+```cpp
+class A { /* ... */ };
+class B : public A { /* ... */ };
+
+void to_json(nlohmann::json& j, const B& b) {
+    nlohmann::to_json(j, static_cast<const A&>(b));
+    // ...
+}
+
+void from_json(const nlohmann::json& j, B& b) {
+    nlohmann::from_json(j, static_cast<A&>(b));
+    // ...
+}
+```
+
+## Notes
+
+!!! info "Prerequisites"
+
+    - Macros 1 and 2 have the same prerequisites of NLOHMANN_DEFINE_TYPE_INTRUSIVE. 
+    - Macros 3 and 3 have the same prerequisites of NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE.
+    - Serialization/deserialization of base types must be defined.
+
+!!! warning "Implementation limits"
+
+    - See Implementation limits for NLOHMANN_DEFINE_TYPE_INTRUSIVE and NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE.
+
+## Examples
+
+Example of `NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE` usage:
+
+```cpp
+class A {
+  double Aa;
+  double Ab;
+  NLOHMANN_DEFINE_TYPE_INTRUSIVE(A, Aa, Ab)
+};
+
+class B : public A {
+  int Ba;
+  int Bb;
+  NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE(B, A, Ba, Bb)
+};
+```
+
+## See also
+
+- [NLOHMANN_DEFINE_TYPE_INTRUSIVE / NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT](nlohmann_define_type_intrusive.md)
+  for similar macros that can be defined _inside_ a non-derived type.
+- [NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE / NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT](nlohmann_define_type_non_intrusive.md)
+  for a similar macros that can be defined _outside_ a non-derived type.
+- [Arbitrary Type Conversions](../../features/arbitrary_types.md) for an overview.
+
+## Version history
+
+1. Added in version 3.11.x.