Advanced serialization macros

[ignus2]

NOTE: Updated macros are in later comments!

#include <map.h> // From: https://github.com/swansontec/map-macro

namespace json {
	using json = nlohmann::json;

	template <class T>
	void get_value(const json& j, const char* n, T& v)
	{
		j.at(n).get_to(v);
	}

	template <class T>
	void get_value(const json& j, const char* n, T& v, const T& d)
	{
		if (const auto it = j.find(n); it != j.end())
			it->get_to(v);
		else
			v = d;
	}
}

// https://www.mikeash.com/pyblog/friday-qa-2015-03-20-preprocessor-abuse-and-optional-parentheses.html
#define PASTE(x, ...) x ## __VA_ARGS__
#define EVALUATING_PASTE(x, ...) PASTE(x, __VA_ARGS__)
#define EXTRACT(...) EXTRACT __VA_ARGS__
#define NOTHING_EXTRACT
#define UNPAREN(x) EVALUATING_PASTE(NOTHING_, EXTRACT x)

// https://stackoverflow.com/a/11763277
#define GET_MACRO(_1,_2,NAME,...) NAME

#define JSON_TO1(f, ...) j[#f] = v.f;
#define JSON_TO_SELECT(f) JSON_TO1(f) // needed for UNPAREN(f) to be evaluated and not stringified as-is
#define JSON_TO(f) JSON_TO_SELECT(UNPAREN(f))

#define JSON_FROM1(f) json::get_value(j, #f, v.f);
#define JSON_FROM2(f, d) json::get_value(j, #f, v.f, d);
#define JSON_FROM_SELECT(...) GET_MACRO(__VA_ARGS__, JSON_FROM2, JSON_FROM1)(__VA_ARGS__)
#define JSON_FROM(f) JSON_FROM_SELECT(UNPAREN(f))

// Usage:
// JSON_SERIALIZE(SomeClass, field1, (field2, "defaultValue"), field3)
#define JSON_SERIALIZE(Type, ...) \
	void to_json(json::json& j, const Type& v) { \
		MAP(JSON_TO, __VA_ARGS__) \
	} \
	void from_json(const json::json& j, Type& v) { \
		MAP(JSON_FROM, __VA_ARGS__) \
	}

The JSON_SERIALIZE() macro above allows specifying default values in case a key does not exist. It is intended as a replacement for NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE().
It needs standard compliant preprocessor, so for MSVC you must enable /Zc:preprocessor for it to work.

Use like so:

struct SomeClass {
    int num;
    int numOptional;
    std::string str;
    std::string strOptional;
};
JSON_SERIALIZE(SomeClass,
    num,
    (numOptional, 123),
    str,
    (strOptional, std::string("missing value"))
)

Feel free to use/enhance it as you want.

[ignus2]

The above can be extended in various ways, for example:

• allowing null values to be deserialized as default values
• special behaviour on null values
• additional parsing flags per field
• etc.

Referencing #2587 the issue there can also be fixed this way.

For example, to allow parsing nulls as default values, one simply needs to modify it like so:

namespace json {
	using json = nlohmann::json;

	template <class T>
	void get_to_allow_null(const json& jv, T& v)
	{
		jv.is_null() ? v = T() : jv.get_to(v);
	}

	template <class T>
	void get_value(const json& j, const char* n, T& v)
	{
		get_to_allow_null(j.at(n), v);
	}

	template <class T>
	void get_value(const json& j, const char* n, T& v, const T& d)
	{
		if (const auto it = j.find(n); it != j.end())
			get_to_allow_null(*it, v);
		else
			v = d;
	}
}

Using the above will create default constructed values in case a null is encountered.

Other extensions are possible, like adding additional traits/flags to individual fields, etc., and modifying json::get_value() above accordingly.

[ignus2]

Updated macros, more generic, only get_value() template functions need to be added above in case more arguments are desired per field. Should work with recent compilers, MSVC included (given /Zc:preprocessor).

// https://www.mikeash.com/pyblog/friday-qa-2015-03-20-preprocessor-abuse-and-optional-parentheses.html
#define PASTE(x, ...) x ## __VA_ARGS__
#define EVALUATING_PASTE(x, ...) PASTE(x, __VA_ARGS__)
#define EXTRACT(...) EXTRACT __VA_ARGS__
#define NOTHING_EXTRACT
#define UNPAREN(x) EVALUATING_PASTE(NOTHING_, EXTRACT x)

#define JSON_TO_V(f, ...) j[#f] = v.f;
#define JSON_TO_EVAL(f) JSON_TO_V(f) // needed for UNPAREN(f) to be evaluated and not stringified as-is
#define JSON_TO(f) JSON_TO_EVAL(UNPAREN(f))

#define JSON_FROM_V(f, ...) json::get_value(j, #f, v.f, ## __VA_ARGS__);
#define JSON_FROM_EVAL(f) JSON_FROM_V(f) // needed for UNPAREN(f) to be evaluated and not stringified as-is
#define JSON_FROM(f) JSON_FROM_EVAL(UNPAREN(f))

// Usage:
// JSON_SERIALIZE(SomeClass, field1, (field2, "defaultValue"), field3)
#define JSON_SERIALIZE(Type, ...) \
	void to_json(json::json& j, const Type& v) { \
		MAP(JSON_TO, __VA_ARGS__) \
	} \
	void from_json(const json::json& j, Type& v) { \
		MAP(JSON_FROM, __VA_ARGS__) \
	}