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  1. #include <iostream>
  2. using namespace std;
  3.  
  4. struct If { If(bool active) : active_(active) { } bool active_; };
  5. struct Else { } _else;
  6. struct Endif { } _endif;
  7.  
  8. class xstream
  9. {
  10.   public:
  11.     xstream(std::ostream& os) : os_(os) { }
  12.  
  13.     template <typename T>
  14.     friend xstream& operator<<(xstream& s, const T& t)
  15.     {
  16.     	if (s.active_) s.os_ << t;
  17.     	return s;
  18.     }
  19.  
  20.   private:
  21.     bool active_;
  22.     std::ostream& os_;
  23. };
  24.  
  25. template <>
  26. xstream& operator<< <If>(xstream& s, const If& _if)
  27. {
  28.     s.active_ = _if.active_;
  29.     return s;
  30. }
  31.  
  32. template <>
  33. xstream& operator<<<Else>(xstream& s, const Else& _else)
  34. {
  35.     s.active_ ^= true;
  36.     return s;
  37. }
  38.  
  39. template <>
  40. xstream& operator<<<Endif>(xstream& s, const Endif& _endif)
  41. {
  42.     s.active_ = true;
  43.     return s;
  44. }
  45.  
  46.  
  47. int main(int argc, const char* argv[])
  48. {
  49. 	xstream xs(std::cout);
  50. 	xs << If(argc <= 1) << argc - 1 << _else << "none" << _endif << '\n';
  51. }
Success #stdin #stdout 0s 3344KB
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https://ideone.com/bw9L6i
language:
C++14 (gcc 8.3)
created:
11 years ago
visibility:
public

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