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  1. #include <stdexcept>
  2. #include <iostream>
  3.  
  4. struct vapour_pressure
  5. {
  6.     enum status_t { nonsense, unknown, known, saturated } ;
  7.  
  8.     vapour_pressure() = default ;
  9.     vapour_pressure( double v) { *this = v ; }
  10.  
  11.     vapour_pressure& operator= ( double v )
  12.     {
  13.         value = v ;
  14.         if( value < 0 ) status = nonsense ;
  15.         // else if ...
  16.         else status = known ;
  17.         return *this ;
  18.     }
  19.  
  20.     operator double() const
  21.     {
  22.         if( status == nonsense ) throw std::domain_error("nonsense") ;
  23.         if( status == unknown ) throw std::domain_error("unknown") ;
  24.  
  25.         return value ;
  26.     }
  27.  
  28.     bool valid() const { return status == known || status == saturated ; }
  29.  
  30.     operator bool () const { return valid() && value ; }
  31.     bool operator! () const { return !bool(*this) ; }
  32.  
  33.     // etc.
  34.  
  35.     private:
  36.         double value ;
  37.         status_t status = unknown ;
  38.         // ...
  39.  
  40.     friend std::ostream& operator<< ( std::ostream& stm, const vapour_pressure& vp )
  41.     {
  42.         if( vp.status == vapour_pressure::unknown ) return stm << "unknown" ;
  43.         else if( vp.status == vapour_pressure::nonsense ) return stm << "nonsense" ;
  44.         else if( vp.status == vapour_pressure::saturated ) return stm << "saturated" ;
  45.         else return stm << vp.value ;
  46.     }
  47. };
  48.  
  49. int main()
  50. {
  51.     vapour_pressure vp ;
  52.     std::cout << "vapour pressure is: " << vp << '\n' ;
  53.     if(vp) { double val = vp ; /* ... */ }
  54.  
  55.     vp = 23.4 ;
  56.     std::cout << "vapour pressure is: " << vp << '\n' ;
  57.     if( vp.valid() ) { double psi = double(vp) * 14.5038 ; /* ... */ }
  58.  
  59.     vp = -3 ;
  60.     std::cout << "vapour pressure is: " << vp << '\n' ;
  61. }
  62.  
Success #stdin #stdout 0s 3340KB
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stdin
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Standard input is empty
stdout
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vapour pressure is: unknown
vapour pressure is: 23.4
vapour pressure is: nonsense
https://ideone.com/J2DlQk
language:
C++14 (gcc 8.3)
created:
11 years ago
visibility:
public

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