Documentation change (#3672)

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

docs/examples/from_json__default_constructible.cpp

@@ -0,0 +1,37 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+namespace ns
+{
+// a simple struct to model a person
+struct person
+{
+    std::string name;
+    std::string address;
+    int age;
+};
+} // namespace ns
+
+namespace ns
+{
+void from_json(const json& j, person& p)
+{
+    j.at("name").get_to(p.name);
+    j.at("address").get_to(p.address);
+    j.at("age").get_to(p.age);
+}
+} // namespace ns
+
+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;
+}

docs/examples/from_json__default_constructible.output

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+Ned Flanders (60) lives in 744 Evergreen Terrace

docs/examples/from_json__non_default_constructible.cpp

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+#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;
+}

docs/examples/from_json__non_default_constructible.output

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+Ned Flanders (60) lives in 744 Evergreen Terrace

docs/examples/nlohmann_json_namespace.cpp

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+#include <iostream>
+#include <nlohmann/json.hpp>
+
+// possible use case: use NLOHMANN_JSON_NAMESPACE instead of nlohmann
+using json = NLOHMANN_JSON_NAMESPACE::json;
+
+// macro needed to output the NLOHMANN_JSON_NAMESPACE as string literal
+#define Q(x) #x
+#define QUOTE(x) Q(x)
+
+int main()
+{
+    std::cout << QUOTE(NLOHMANN_JSON_NAMESPACE) << std::endl;
+}

docs/examples/nlohmann_json_namespace.output

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+nlohmann::json_v3_11_1

docs/examples/nlohmann_json_namespace_begin.c++17.cpp

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+#include <iostream>
+#include <optional>
+#include <nlohmann/json.hpp>
+
+// partial specialization (see https://json.nlohmann.me/features/arbitrary_types/)
+NLOHMANN_JSON_NAMESPACE_BEGIN
+template <typename T>
+struct adl_serializer<std::optional<T>>
+{
+    static void to_json(json& j, const std::optional<T>& opt)
+    {
+        if (opt == std::nullopt)
+        {
+            j = nullptr;
+        }
+        else
+        {
+            j = *opt;
+        }
+    }
+};
+NLOHMANN_JSON_NAMESPACE_END
+
+int main()
+{
+    std::optional<int> o1 = 1;
+    std::optional<int> o2 = std::nullopt;
+
+    NLOHMANN_JSON_NAMESPACE::json j;
+    j.push_back(o1);
+    j.push_back(o2);
+    std::cout << j << std::endl;
+}

docs/examples/nlohmann_json_namespace_begin.c++17.output

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+[1,null]

docs/examples/to_json.cpp

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

docs/examples/to_json.output

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