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  1. #include <cstddef>
  2.  
  3. #include <iostream>
  4. #include <memory>
  5. #include <type_traits>
  6.  
  7. template <typename, size_t> class FastFunc;
  8.  
  9. template <typename R, typename... Args, size_t Size>
  10. class FastFunc<R(Args...), Size> {
  11. public:
  12.     template <typename F>
  13.     FastFunc(F f): handler(&Get<F>()) {
  14.         new (&storage) F(std::move(f));
  15.     }
  16.  
  17.     ~FastFunc() {
  18.         handler->destroy(&storage);
  19.     }
  20.  
  21.     R operator()(Args&&... args) {
  22.       return handler->apply(&storage, std::forward<Args>(args)...);
  23.     }
  24.  
  25. private:
  26.     using Storage = typename std::aligned_storage<Size, alignof(max_align_t)>::type;
  27.  
  28.     struct Handler {
  29.         R (*apply)(void*, Args&&...);
  30.         void (*destroy)(void*);
  31.     }; // struct Handler
  32.  
  33.     template <typename F>
  34.     static R Apply(void* f, Args&&... args) {
  35.         (*reinterpret_cast<F*>(f))(std::forward<Args>(args)...);
  36.     }
  37.  
  38.     template <typename F>
  39.     static void Destroy(void* f) {
  40.     	reinterpret_cast<F*>(f)->~F();
  41.     }
  42.  
  43.     template <typename F>
  44.     Handler const& Get() {
  45.         static Handler const H = { &Apply<F>, &Destroy<F> };
  46.         return H;
  47.     } // Get
  48.  
  49.     Handler const* handler;
  50.     Storage storage;
  51. }; // class FastFunc
  52.  
  53. int main() {
  54. 	FastFunc<void(), 32> stateless = []() { std::cout << "stateless\n"; };
  55. 	stateless();
  56.  
  57. 	bool b = true;
  58. 	FastFunc<void(), 32> stateful = [&b]() { std::cout << "stateful: " << b << "\n"; };
  59. 	stateful();
  60.  
  61. 	b = false;
  62. 	stateful();
  63.  
  64. 	return 0;
  65. }
Success #stdin #stdout 0s 3340KB
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Standard input is empty
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stateless
stateful: 1
stateful: 0
https://ideone.com/oBdzxc
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
public

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