Add serialization-only user defined type macros (#3816)

docs/examples/nlohmann_define_type_intrusive_only_serialize_explicit.cpp

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+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+using namespace nlohmann::literals;
+
+namespace ns
+{
+class person
+{
+  private:
+    std::string name = "John Doe";
+    std::string address = "123 Fake St";
+    int age = -1;
+
+  public:
+    // No default constructor
+    person(std::string name_, std::string address_, int age_)
+        : name(std::move(name_)), address(std::move(address_)), age(age_)
+    {}
+
+    friend void to_json(nlohmann::json& nlohmann_json_j, const person& nlohmann_json_t)
+    {
+        nlohmann_json_j["name"] = nlohmann_json_t.name;
+        nlohmann_json_j["address"] = nlohmann_json_t.address;
+        nlohmann_json_j["age"] = nlohmann_json_t.age;
+    }
+};
+} // namespace ns
+
+int main()
+{
+    ns::person p = {"Ned Flanders", "744 Evergreen Terrace", 60};
+
+    // serialization: person -> json
+    json j = p;
+    std::cout << "serialization: " << j << std::endl;
+}

docs/examples/nlohmann_define_type_intrusive_only_serialize_explicit.output

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+serialization: {"address":"744 Evergreen Terrace","age":60,"name":"Ned Flanders"}

docs/examples/nlohmann_define_type_intrusive_only_serialize_macro.cpp

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+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+using namespace nlohmann::literals;
+
+namespace ns
+{
+class person
+{
+  private:
+    std::string name = "John Doe";
+    std::string address = "123 Fake St";
+    int age = -1;
+
+  public:
+    // No default constructor
+    person(std::string name_, std::string address_, int age_)
+        : name(std::move(name_)), address(std::move(address_)), age(age_)
+    {}
+
+    NLOHMANN_DEFINE_TYPE_INTRUSIVE(person, name, address, age)
+};
+} // namespace ns
+
+int main()
+{
+    ns::person p = {"Ned Flanders", "744 Evergreen Terrace", 60};
+
+    // serialization: person -> json
+    json j = p;
+    std::cout << "serialization: " << j << std::endl;
+}

docs/examples/nlohmann_define_type_intrusive_only_serialize_macro.output

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+serialization: {"address":"744 Evergreen Terrace","age":60,"name":"Ned Flanders"}

docs/examples/nlohmann_define_type_non_intrusive_only_serialize_explicit.cpp

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+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+using namespace nlohmann::literals;
+
+namespace ns
+{
+struct person
+{
+    std::string name;
+    std::string address;
+    int age;
+};
+
+void to_json(nlohmann::json& nlohmann_json_j, const person& nlohmann_json_t)
+{
+    nlohmann_json_j["name"] = nlohmann_json_t.name;
+    nlohmann_json_j["address"] = nlohmann_json_t.address;
+    nlohmann_json_j["age"] = nlohmann_json_t.age;
+}
+} // namespace ns
+
+int main()
+{
+    ns::person p = {"Ned Flanders", "744 Evergreen Terrace", 60};
+
+    // serialization: person -> json
+    json j = p;
+    std::cout << "serialization: " << j << std::endl;
+}

docs/examples/nlohmann_define_type_non_intrusive_only_serialize_explicit.output

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+serialization: {"address":"744 Evergreen Terrace","age":60,"name":"Ned Flanders"}

docs/examples/nlohmann_define_type_non_intrusive_only_serialize_macro.cpp

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+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+using namespace nlohmann::literals;
+
+namespace ns
+{
+struct person
+{
+    std::string name;
+    std::string address;
+    int age;
+};
+
+NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE(person, name, address, age)
+} // namespace ns
+
+int main()
+{
+    ns::person p = {"Ned Flanders", "744 Evergreen Terrace", 60};
+
+    // serialization: person -> json
+    json j = p;
+    std::cout << "serialization: " << j << std::endl;
+}

docs/examples/nlohmann_define_type_non_intrusive_only_serialize_macro.output

@@ -0,0 +1 @@
+serialization: {"address":"744 Evergreen Terrace","age":60,"name":"Ned Flanders"}