Documentation change (#3672)

Co-authored-by: Florian Albrechtskirchinger <falbrechtskirchinger@gmail.com>

docs/examples/from_json__non_default_constructible.cpp

@@ -0,0 +1,53 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+namespace ns
+{
+// a simple struct to model a person (not default constructible)
+struct person
+{
+    person(std::string n, std::string a, int aa)
+        : name(std::move(n)), address(std::move(a)), age(aa)
+    {}
+
+    std::string name;
+    std::string address;
+    int age;
+};
+} // namespace ns
+
+namespace nlohmann
+{
+template <>
+struct adl_serializer<ns::person>
+{
+    static ns::person from_json(const json& j)
+    {
+        return {j.at("name"), j.at("address"), j.at("age")};
+    }
+
+    // Here's the catch! You must provide a to_json method! Otherwise, you
+    // will not be able to convert person to json, since you fully
+    // specialized adl_serializer on that type
+    static void to_json(json& j, ns::person p)
+    {
+        j["name"] = p.name;
+        j["address"] = p.address;
+        j["age"] = p.age;
+    }
+};
+} // namespace nlohmann
+
+int main()
+{
+    json j;
+    j["name"] = "Ned Flanders";
+    j["address"] = "744 Evergreen Terrace";
+    j["age"] = 60;
+
+    auto p = j.get<ns::person>();
+
+    std::cout << p.name << " (" << p.age << ") lives in " << p.address << std::endl;
+}