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  1. #include <iostream>
  2.  
  3. template<class T>using type=T;
  4. template<class F>
  5. struct callback_t;
  6.  
  7. template<class F, class Sig>
  8. struct cast_helper_pvoid_last;
  9. template<class F, class R, class... Args>
  10. struct cast_helper_pvoid_last<F, R(Args...)> {
  11.   type<R(*)(Args..., void*)> operator()() const {
  12.     return [](Args... args, void* pvoid)->R {
  13.       auto* callback = static_cast<callback_t<F>*>(pvoid);
  14.       return callback->f( std::forward<Args>(args)... );
  15.     };
  16.   }
  17. };
  18.  
  19. template<class F>
  20. struct callback_t {
  21.   F f;
  22.   void* pvoid() { return this; }
  23.  
  24.   template<class Sig>
  25.   auto pvoid_at_end()->decltype( cast_helper_pvoid_last<F, Sig>{}() ) {
  26.     return cast_helper_pvoid_last<F,Sig>{}();
  27.   }
  28. };
  29. template<class T>using decay_t=typename std::decay<T>::type;
  30. template<class F>
  31. callback_t<decay_t<F>> make_callback( F&& f ) { return {std::forward<F>(f)}; }
  32.  
  33. int main() {
  34. 	int x = 3;
  35. 	auto callback = make_callback( [&]( int y ) { return x+y; } );
  36. 	int (*func)(int, void*) = callback.pvoid_at_end<int(int)>();
  37. 	std::cout << func( 1, callback.pvoid() ) << "\n";
  38. }
Success #stdin #stdout 0s 3296KB
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stdin 
Standard input is empty
stdout 
4
https://ideone.com/4fNzK2
language:
C++14 (gcc 8.3)
created:
11 years ago
visibility:
public

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