Ttdfjt

I wanted to detect if I have a member in a simple POD struct and after some searching and merging some methods I found on the web I've come up with this solution:

#include <iostream>

template<class...Fs>
struct funcs_t;

template<class F0, class...Fs>
struct funcs_t<F0, Fs...>: F0, funcs_t<Fs...> 
 funcs_t(F0 f0, Fs... fs):
 F0(std::move(f0)), funcs_t<Fs...>(std::move(fs)...)
 
 using F0::operator();
 using funcs_t<Fs...>::operator();
;

template<class F>
struct funcs_t<F>:F 
 funcs_t(F f) : F(std::move(f)) ;
 using F::operator();
;

template<class...Fs>
funcs_t<std::decay_t<Fs>...> funcs(Fs&&...fs) 
 return std::forward<Fs>(fs)...;


#define HAS_MEMBER(cls, memb)
 []()
 auto with_memb = [](auto &&A, int x)
 -> decltype(typename std::remove_reference<decltype(A)>::type().memb, std::true_type())
 
 return std::true_type();
 ;
 auto with_no_memb = [](auto &&A, float x) -> std::false_type 
 return std::false_type();
 ;
 auto has_memb = funcs(with_memb, with_no_memb);
 return has_memb(cls(), 1);
 ()

struct A 
 int a;
 int b;
;

int main(int argc, char const *argv[])

 std::cout << HAS_MEMBER(A, b) << std::endl;
 return 0;

1. 
   

   Your detector fails in the face of overloading and templating. Also, it will only detect (static) member-variables and (static) member-functions. While you can extend it to types (and type-aliases), accepting templates and overloading would need better language-provided reflection-facilities.

   
   
   

   Anyway, pure existence is generally uninteresting, supported operations count.

   
   


2. 
   

   funcs() and funcs_t are nearly a generally useful abstraction.

   
   
   

   Just use perfect forwarding instead of by-value and std::move(). Allowing for all callables, including final classes, function-pointers, member-function-pointers, and the same wrapped in a std::reference_wrapper would admittedly add significant amounts of code.

   
   
   

   A good name would be overloaded.

   
   



3. HAS_MEMBER needlessly depends on default-constructing the passed class. Fix that by using decltype, std::declval() and unevaluated contexts.




4. If you don't use an argument, don't name it. Specifically for main(), just don't ask for it.




5. Don't use std::endl. In the rare cases you actually need to flush manually, be explicit and use std::flush. Nearly always you are just crippling performance.




6. return 0; is implicit for main(). Make of that what you will.