add built-in support for custom visitation contexts

[cbeck88]

this avoids the complexity of introducing proxy objects (and handling copies of these) when one wants to visit the same object in multiple ways

follows plan in issue #34

building but still needs tests

[cbeck88]

this needs docu, then i'm probably happy to ship it

[Sven-vh]

This is perfect, thank you for taking your time and implementing this!

I tried this branch with my Transform test and it works perfectly! I like that the VISITABLE_STRUCT macro uses void as default context. Makes it super easy to do stuff like this, where you take the context as template parameter.

struct SerializeContext {};
struct PrintContext {};
struct ScriptingContext {};

struct Transform {
private:
	BEFRIEND_VISITABLE();
	glm::vec3 m_position;
	glm::vec3 m_scale;
	glm::quat m_rotation;
};

VISITABLE_STRUCT(Transform, m_position, m_scale, m_rotation);
VISITABLE_STRUCT_IN_CONTEXT(SerializeContext, Transform, m_position);
VISITABLE_STRUCT_IN_CONTEXT(PrintContext, Transform, m_scale);
VISITABLE_STRUCT_IN_CONTEXT(ScriptingContext, Transform, m_rotation);

struct debug_printer {
	template <typename T>
	void operator()(const char* name, const T&) {
		std::cout << name << std::endl;
	}
};

//template <typename T, typename Context = void>
// Swapped Context and T so I don't have to specify T when calling print_properties
template <typename Context = void, typename T>
void print_properties(const T& obj) {
	visit_struct::context<Context>::for_each(obj, debug_printer{});
}

int main() {
	Transform transform;
	print_properties(transform); // uses void, thus default macro, thus: m_position, m_scale, m_rotation
	print_properties<SerializeContext>(transform); // m_position
	print_properties<PrintContext>(transform); // m_scale
	print_properties<ScriptingContext>(transform); // m_rotation
}

I'm planning on using it a lot when making custom engines. Probably going to use it to differentiate between:

• Serializing
• Debug printing
• Properties in scripting
• Properties in inspector

Limitations are endless. Thanks again for this feature and your time!

[Sven-vh]

Like I said, ran my Transfrom test and everything works.
Tested with msvc on C++ 14.

Errors are also super nice. Misspelling or having a duplicate context give clear errors that are easy to debug/know what is wrong. Awesome job!

[cbeck88]

Thank you for testing!

I still need to write some docs and maybe extend the tests, then will merge