fun with pointers to member functions

member_fun.cpp

// https://godbolt.org/z/3s1r3K4rs
#include <string>

// For c++ versions earlier than 17
#if __cplusplus < 201703L
// See https://isocpp.org/wiki/faq/pointers-to-members#macro-for-ptr-to-memfn
// and https://isocpp.org/wiki/faq/big-picture#use-evil-things-sometimes
#define CALL_MEMBER_FN(object,ptrToMember)  (((object).*(ptrToMember))())
#else
#include <functional>
#define CALL_MEMBER_FN(object, ptrToMember) (std::invoke(ptrToMember, object))
#endif

struct ThingHolder {
    std::string thing1;
    std::string thing2;
    std::string thingN;
};

class Builder {
public:
    Builder& setThing1(std::string aThing1)
    {
        mThing.thing1 = std::move(aThing1);
        return *this;
    }

    Builder& setThing2(std::string aThing2)
    {
        mThing.thing2 = std::move(aThing2);
        return *this;
    }

    Builder& setThingN(std::string aThingN)
    {
        mThing.thingN = std::move(aThingN);
        return *this;
    }

    const ThingHolder build() const
    {
        return mThing;
    }
private:
    ThingHolder mThing;
};


struct Getter {
    std::string thing1;
    std::string thing2;
    std::string thingN;
    
    const std::string& GetThing1() const
    {
        return thing1;
    }
    
    const std::string& GetThing2() const
    {
        return thing2;
    }

    const std::string& GetThingN() const
    {
        return thingN;
    } 
};

int main() {
    using GetterMemFn = const std::string& (Getter::*)(void) const;

    Getter objA;
    objA.thing1 = "1";
    Getter objB;
    objB.thing2 = "123";
    const auto magicFunction = [&objA, &objB](const GetterMemFn& memFn) {
        return !CALL_MEMBER_FN(objA, memFn).empty() ?
               CALL_MEMBER_FN(objA, memFn) :
               CALL_MEMBER_FN(objB, memFn);
    };
    
    const auto thingHolder {
        [&]{
            return Builder{}
                .setThing1(magicFunction(&Getter::GetThing1))
                .setThing2(magicFunction(&Getter::GetThing2))
                .setThingN(magicFunction(&Getter::GetThingN))
                .build();
        }()
    };

    return thingHolder.thing1.size();
}