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  1. #include <iostream>
  2.  
  3. template <class T>
  4. class Maybe {
  5.     bool is_ = false;
  6.     T value_;
  7.  
  8.   public:
  9.     constexpr Maybe() = default;
  10.     constexpr Maybe(T value) : is_(true), value_(value) {}
  11.  
  12.     constexpr bool is() const { return is_; }
  13. };
  14.  
  15. Maybe<int> not_set_;
  16. Maybe<int> is_set_(2);
  17.  
  18. template <const Maybe<int>& parm>
  19. struct Test {
  20.     void say() const {
  21.         std::cout << "parm is " << (parm.is() ? "set" : "not set") << ".\n";
  22.     }
  23. };
  24.  
  25. int main() {
  26.     Test<not_set_> not_set;
  27.     Test<is_set_> is_set;
  28.  
  29.     not_set.say();
  30.     is_set.say();
  31. }
  32.  
Success #stdin #stdout 0s 3340KB
comments (?)
stdin
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Standard input is empty
stdout
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parm is not set.
parm is set.
https://ideone.com/nvTwKd
language:
C++14 (gcc 8.3)
created:
10 years ago
visibility:
public

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