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  1. #include <functional>
  2. #include <iostream>
  3. #include <string>
  4.  
  5. template<typename T>
  6. class lazy {
  7. //TYPES
  8.     typedef typename std::aligned_storage<
  9.             sizeof(T), 
  10.             std::alignment_of<T>::value
  11.         >::type uninit_t;
  12. //MEMBERS
  13.     std::function<T()> f;
  14.     uninit_t value;
  15.     bool cached;
  16. public:
  17.     lazy(std::function<T()> f_) 
  18.     :     f      { std::move(f_) }
  19.     ,     cached { false         }
  20.     {     }
  21.  
  22.     T& get() {
  23.         if(!cached) { 
  24.             new(&value) T(f());
  25.             cached=true;
  26.         }   
  27.         return reinterpret_cast<T&>(value); 
  28.     }   
  29.     ~lazy() {
  30.         get().~T();
  31.     }   
  32. };
  33.  
  34.  
  35.  
  36. std::string make_big_string() {
  37.     std::cerr << "making string\n";
  38.     return "hello, world";
  39. }
  40.  
  41.  
  42. int main() {
  43.     lazy<std::string> l_str(make_big_string); 
  44.     std::cerr << "get1\n";
  45.     std::cout << l_str.get() << '\n';
  46.     std::cerr << "get1\n";
  47.     std::cout << l_str.get() << '\n';
  48. }
Success #stdin #stdout 0s 3016KB
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Standard input is empty
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hello, world
hello, world
https://ideone.com/dhuUg
language:
C++14 (gcc 8.3)
created:
11 years ago
visibility:
public

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