Add macros NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT and ..._NON_INTRUSIVE_WITH_DEFAULT (#3143)

* Added new macros NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT and NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT.

* Updated docs for NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT and NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT accordingly

* Rephrased docs for NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT and NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT

* Updated docs for NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT and NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT

* Renamed default_obj in to avoid name clashes

* Added test for serialization of default constructed object

* Add const to getters for macro tests

Co-authored-by: Chaoya Li <harry75369@gmail.com>

doc/mkdocs/docs/features/arbitrary_types.md

@@ -85,29 +85,31 @@ Some important things:
 
 If you just want to serialize/deserialize some structs, the `to_json`/`from_json` functions can be a lot of boilerplate.
 
-There are two macros to make your life easier as long as you (1) want to use a JSON object as serialization and (2) want to use the member variable names as object keys in that object:
+There are four macros to make your life easier as long as you (1) want to use a JSON object as serialization and (2) want to use the member variable names as object keys in that object:
 
-- `NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(name, member1, member2, ...)` is to be defined inside the namespace of the class/struct to create code for.
-- `NLOHMANN_DEFINE_TYPE_INTRUSIVE(name, member1, member2, ...)` is to be defined inside the class/struct to create code for. This macro can also access private members.
+- `NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(name, member1, member2, ...)` is to be defined inside the namespace of the class/struct to create code for. It will throw an exception in `from_json()` due to a missing value in the JSON object.
+- `NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT(name, member1, member2, ...)` is to be defined inside the namespace of the class/struct to create code for. It will not throw an exception in `from_json()` due to a missing value in the JSON object, but fills in values from object which is default-constructed by the type.
+- `NLOHMANN_DEFINE_TYPE_INTRUSIVE(name, member1, member2, ...)` is to be defined inside the class/struct to create code for. This macro can also access private members. It will throw an exception in `from_json()` due to a missing value in the JSON object.
+- `NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT(name, member1, member2, ...)` is to be defined inside the class/struct to create code for. This macro can also access private members. It will not throw an exception in `from_json()` due to a missing value in the JSON object, but fills in values from object which is default-constructed by the type.
 
-In both macros, the first parameter is the name of the class/struct, and all remaining parameters name the members.
+In all macros, the first parameter is the name of the class/struct, and all remaining parameters name the members. You can read more docs about them starting from [here](macros.md#nlohmann_define_type_intrusivetype-member).
 
 !!! note
 
-    At most 64 member variables can be passed to `NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE` or `NLOHMANN_DEFINE_TYPE_INTRUSIVE`.
+    At most 64 member variables can be passed to these macros.
 
 ??? example
 
     The `to_json`/`from_json` functions for the `person` struct above can be created with:
-    
+
     ```cpp
     namespace ns {
         NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(person, name, address, age)
     }
     ```
-    
+
     Here is an example with private members, where `NLOHMANN_DEFINE_TYPE_INTRUSIVE` is needed:
-    
+
     ```cpp
     namespace ns {
         class address {
@@ -115,7 +117,7 @@ In both macros, the first parameter is the name of the class/struct, and all rem
             std::string street;
             int housenumber;
             int postcode;
-            
+
           public:
             NLOHMANN_DEFINE_TYPE_INTRUSIVE(address, street, housenumber, postcode)
         };

doc/mkdocs/docs/features/macros.md

@@ -186,12 +186,12 @@ When defined to `0`, implicit conversions are switched off. By default, implicit
 ??? example
 
     This is an example for an implicit conversion:
-    
+
     ```cpp
     json j = "Hello, world!";
     std::string s = j;
     ```
-    
+
     When `JSON_USE_IMPLICIT_CONVERSIONS` is defined to `0`, the code above does no longer compile. Instead, it must be
     written like this:
 
@@ -220,6 +220,10 @@ The first parameter is the name of the class/struct, and all remaining parameter
 
 See [Simplify your life with macros](arbitrary_types.md#simplify-your-life-with-macros) for an example.
 
+## `NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT(type, member...)`
+
+This macro is similar to `NLOHMANN_DEFINE_TYPE_INTRUSIVE`. It will not throw an exception in `from_json()` due to a missing value in the JSON object, but can throw due to a mismatched type. In order to support that it requires that the type be default constructible. The `from_json()` function default constructs an object and uses its values as the defaults when calling the `value()` function.
+
 ## `NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(type, member...)`
 
 This macro can be used to simplify the serialization/deserialization of types if (1) want to use a JSON object as
@@ -231,6 +235,10 @@ first parameter is the name of the class/struct, and all remaining parameters na
 
 See [Simplify your life with macros](arbitrary_types.md#simplify-your-life-with-macros) for an example.
 
+## `NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT(type, member...)`
+
+This macro is similar to `NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE`. It will not throw an exception in `from_json()` due to a missing value in the JSON object, but can throw due to a mismatched type. In order to support that it requires that the type be default constructible. The `from_json()` function default constructs an object and uses its values as the defaults when calling the `value()` function.
+
 ## `NLOHMANN_JSON_SERIALIZE_ENUM(type, ...)`
 
 This macro simplifies the serialization/deserialization of enum types. See

include/nlohmann/detail/macro_scope.hpp

@@ -334,6 +334,7 @@
 
 #define NLOHMANN_JSON_TO(v1) nlohmann_json_j[#v1] = nlohmann_json_t.v1;
 #define NLOHMANN_JSON_FROM(v1) nlohmann_json_j.at(#v1).get_to(nlohmann_json_t.v1);
+#define NLOHMANN_JSON_FROM_WITH_DEFAULT(v1) nlohmann_json_t.v1 = nlohmann_json_j.value(#v1, nlohmann_json_default_obj.v1);
 
 /*!
 @brief macro
@@ -344,6 +345,10 @@
     friend void to_json(nlohmann::json& nlohmann_json_j, const Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) } \
     friend void from_json(const nlohmann::json& nlohmann_json_j, Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM, __VA_ARGS__)) }
 
+#define NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT(Type, ...)  \
+    friend void to_json(nlohmann::json& nlohmann_json_j, const Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) } \
+    friend void from_json(const nlohmann::json& nlohmann_json_j, Type& nlohmann_json_t) { Type nlohmann_json_default_obj; NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM_WITH_DEFAULT, __VA_ARGS__)) }
+
 /*!
 @brief macro
 @def NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE
@@ -353,6 +358,10 @@
     inline void to_json(nlohmann::json& nlohmann_json_j, const Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) } \
     inline void from_json(const nlohmann::json& nlohmann_json_j, Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM, __VA_ARGS__)) }
 
+#define NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT(Type, ...)  \
+    inline void to_json(nlohmann::json& nlohmann_json_j, const Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) } \
+    inline void from_json(const nlohmann::json& nlohmann_json_j, Type& nlohmann_json_t) { Type nlohmann_json_default_obj; NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM_WITH_DEFAULT, __VA_ARGS__)) }
+
 
 // inspired from https://stackoverflow.com/a/26745591
 // allows to call any std function as if (e.g. with begin):

single_include/nlohmann/json.hpp

@@ -2631,6 +2631,7 @@ using is_detected_convertible =
 
 #define NLOHMANN_JSON_TO(v1) nlohmann_json_j[#v1] = nlohmann_json_t.v1;
 #define NLOHMANN_JSON_FROM(v1) nlohmann_json_j.at(#v1).get_to(nlohmann_json_t.v1);
+#define NLOHMANN_JSON_FROM_WITH_DEFAULT(v1) nlohmann_json_t.v1 = nlohmann_json_j.value(#v1, nlohmann_json_default_obj.v1);
 
 /*!
 @brief macro
@@ -2641,6 +2642,10 @@ using is_detected_convertible =
     friend void to_json(nlohmann::json& nlohmann_json_j, const Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) } \
     friend void from_json(const nlohmann::json& nlohmann_json_j, Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM, __VA_ARGS__)) }
 
+#define NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT(Type, ...)  \
+    friend void to_json(nlohmann::json& nlohmann_json_j, const Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) } \
+    friend void from_json(const nlohmann::json& nlohmann_json_j, Type& nlohmann_json_t) { Type nlohmann_json_default_obj; NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM_WITH_DEFAULT, __VA_ARGS__)) }
+
 /*!
 @brief macro
 @def NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE
@@ -2650,6 +2655,10 @@ using is_detected_convertible =
     inline void to_json(nlohmann::json& nlohmann_json_j, const Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) } \
     inline void from_json(const nlohmann::json& nlohmann_json_j, Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM, __VA_ARGS__)) }
 
+#define NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT(Type, ...)  \
+    inline void to_json(nlohmann::json& nlohmann_json_j, const Type& nlohmann_json_t) { NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_TO, __VA_ARGS__)) } \
+    inline void from_json(const nlohmann::json& nlohmann_json_j, Type& nlohmann_json_t) { Type nlohmann_json_default_obj; NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(NLOHMANN_JSON_FROM_WITH_DEFAULT, __VA_ARGS__)) }
+
 
 // inspired from https://stackoverflow.com/a/26745591
 // allows to call any std function as if (e.g. with begin):

test/src/unit-udt_macro.cpp

@@ -59,6 +59,42 @@ class person_with_private_data
     NLOHMANN_DEFINE_TYPE_INTRUSIVE(person_with_private_data, age, name, metadata)
 };
 
+class person_with_private_data_2
+{
+  private:
+    std::string name{};
+    int age = 0;
+    json metadata = nullptr;
+
+  public:
+    bool operator==(const person_with_private_data_2& rhs) const
+    {
+        return name == rhs.name && age == rhs.age && metadata == rhs.metadata;
+    }
+
+    person_with_private_data_2() = default;
+    person_with_private_data_2(std::string name_, int age_, json metadata_)
+        : name(std::move(name_))
+        , age(age_)
+        , metadata(std::move(metadata_))
+    {}
+
+    std::string getName() const
+    {
+        return name;
+    }
+    int getAge() const
+    {
+        return age;
+    }
+    json getMetadata() const
+    {
+        return metadata;
+    }
+
+    NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT(person_with_private_data_2, age, name, metadata)
+};
+
 class person_without_private_data_1
 {
   public:
@@ -103,6 +139,41 @@ class person_without_private_data_2
 
 NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(person_without_private_data_2, age, name, metadata)
 
+class person_without_private_data_3
+{
+  public:
+    std::string name{};
+    int age = 0;
+    json metadata = nullptr;
+
+    bool operator==(const person_without_private_data_3& rhs) const
+    {
+        return name == rhs.name && age == rhs.age && metadata == rhs.metadata;
+    }
+
+    person_without_private_data_3() = default;
+    person_without_private_data_3(std::string name_, int age_, json metadata_)
+        : name(std::move(name_))
+        , age(age_)
+        , metadata(std::move(metadata_))
+    {}
+
+    std::string getName() const
+    {
+        return name;
+    }
+    int getAge() const
+    {
+        return age;
+    }
+    json getMetadata() const
+    {
+        return metadata;
+    }
+};
+
+NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT(person_without_private_data_3, age, name, metadata)
+
 class person_with_private_alphabet
 {
   public:
@@ -231,7 +302,7 @@ NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(person_with_public_alphabet, a, b, c, d, e, f
 
 } // namespace persons
 
-TEST_CASE_TEMPLATE("Serialization/deserialization via NLOHMANN_DEFINE_TYPE_INTRUSIVE", T,
+TEST_CASE_TEMPLATE("Serialization/deserialization via NLOHMANN_DEFINE_TYPE_INTRUSIVE and NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE", T,
                    persons::person_with_private_data,
                    persons::person_without_private_data_1,
                    persons::person_without_private_data_2)
@@ -257,6 +328,40 @@ TEST_CASE_TEMPLATE("Serialization/deserialization via NLOHMANN_DEFINE_TYPE_INTRU
     }
 }
 
+TEST_CASE_TEMPLATE("Serialization/deserialization via NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT and NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT", T,
+                   persons::person_with_private_data_2,
+                   persons::person_without_private_data_3)
+{
+    SECTION("person with default values")
+    {
+        // serialization of default constructed object
+        T p0;
+        CHECK(json(p0).dump() == "{\"age\":0,\"metadata\":null,\"name\":\"\"}");
+
+        // serialization
+        T p1("Erik", 1, {{"haircuts", 2}});
+        CHECK(json(p1).dump() == "{\"age\":1,\"metadata\":{\"haircuts\":2},\"name\":\"Erik\"}");
+
+        // deserialization
+        auto p2 = json(p1).get<T>();
+        CHECK(p2 == p1);
+
+        // roundtrip
+        CHECK(T(json(p1)) == p1);
+        CHECK(json(T(json(p1))) == json(p1));
+
+        // check default value in case of missing field
+        json j = json(p1);
+        j.erase("name");
+        j.erase("age");
+        j.erase("metadata");
+        T p3 = j.get<T>();
+        CHECK(p3.getName() == "");
+        CHECK(p3.getAge() == 0);
+        CHECK(p3.getMetadata() == nullptr);
+    }
+}
+
 TEST_CASE_TEMPLATE("Serialization/deserialization of classes with 26 public/private member variables via NLOHMANN_DEFINE_TYPE_INTRUSIVE and NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE", T,
                    persons::person_with_private_alphabet,
                    persons::person_with_public_alphabet)