Variadic

wrapper.cpp

#include<iostream>
// Needs C++14 for auto return type...
struct ST{
  void theFn(int a){std::cout<<a<<std::endl;}
  double theOtherFn(int a, double b){std::cout<<a<<" "<<b<<std::endl; return 0.0;}
  void fnByRef(double & a){std::cout<<a<<std::endl;}
};
class wrapper{
  ST theVal;
public:
  //Forward
  template<typename fn, typename ...Args>
  auto invoke(fn callable, Args&&... args){ // Universal reference
    return (theVal.*callable)(std::forward<Args>(args)...); // std::forward helps preserve reference, const etc
  }
};

auto call_theFn(wrapper & theW, int theVal){
  theW.invoke(&ST::theFn, theVal);
}

int main(){

  wrapper myW;

  auto fn = &ST::theFn;
  myW.invoke(fn, 10); //Prints 10

  call_theFn(myW, 7); //Prints 7

  auto fn2 = &ST::theOtherFn;
  myW.invoke(fn2, 1.0, 2.0); //Prints 1 2

  auto fn3 = &ST::fnByRef;
  //myW.invoke(fn3, 3.4); //Invalid - can't bind a value to a non-const reference
  double x = 1.1;
  myW.invoke(fn3, x);
  
}