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  1. #include <iostream>
  2.  
  3. // custom struct to ensure functions are called in the correct order
  4. template<int N>
  5. struct Precedence : public Precedence<N - 1> {};
  6. template<>
  7. struct Precedence<0> {};
  8.  
  9. // The main printer that delegates function calls of other classes.
  10. // Its 'print' function accepts classes that have defined:
  11. // - void print_time(int mins, double secs)
  12. // - void print_time(double total_secs)
  13. // - void print_time(int mins)
  14. // or none of the above.
  15. class PrinterDelegator {
  16. public:
  17.     // The Precedence variable ensures the function with the highest 'N' value
  18.     // gets called. If the function with the current 'N' value is ill-formed,
  19.     // the Precedence struct will cast to its next highest base class until
  20.     // a properly formed function is available.
  21.     template<typename T>
  22.     void print(const T &printer) {
  23.         print(printer, p_);
  24.     }
  25.  
  26. private:
  27.     // these would all actually be incrementing
  28.     int mins_{3};
  29.     double secs_{22.5};
  30.     double total_secs_{202.5};
  31.  
  32.     Precedence<3> p_{};
  33.  
  34.     // The "best" function. Call this if available
  35.     template<typename T>
  36.     auto print(const T &printer, const Precedence<3>&) -> decltype(printer.print_time(mins_, secs_)) {
  37.         printer.print_time(mins_, secs_);
  38.     }
  39.     // Other available functions ordered by importance
  40.     template<typename T>
  41.     auto print(const T &printer, const Precedence<2>&) -> decltype(printer.print_time(total_secs_)) {
  42.         printer.print_time(total_secs_);
  43.     }
  44.     template<typename T>
  45.     auto print(const T &printer, const Precedence<1>&) -> decltype(printer.print_time(mins_)) {
  46.         printer.print_time(mins_);
  47.     }
  48.     // default empty definition allowing for classes that haven't defined 'print_time'
  49.     template<typename T>
  50.     auto print(const T &printer, const Precedence<0>&) -> decltype(void()) {
  51.         std::cout << "nothing" << std::endl;
  52.     }
  53. };
  54.  
  55. // class with all possible functions. Only the "best"
  56. // one will get called: 'void print_time(int, double)'
  57. class VerbosePrinter {
  58. public:
  59.     void print_time(int mins, double secs) const {
  60.         print_time(mins);
  61.         print_time(secs);
  62.     }
  63.     void print_time(double total_secs) const {
  64.         std::cout << total_secs << "s" << std::endl;
  65.     }
  66.     void print_time(int mins) const {
  67.         std::cout << mins << "m" << std::endl;
  68.     }
  69. };
  70.  
  71. // Class with two of the function options. The best one here is
  72. // 'void print_time(double)' so that one will be called
  73. class SinglePrinter {
  74. public:
  75.     void print_time(double total_secs) const {
  76.         std::cout << total_secs << "s" << std::endl;
  77.     }
  78.     void print_time(int mins) const {
  79.         std::cout << mins << "m" << std::endl;
  80.     }
  81. };
  82.  
  83. // Empty class that can still be used even though no functions are defined
  84. class EmptyPrinter {};
  85.  
  86.  
  87. int main() {
  88.     PrinterDelegator pd;
  89.  
  90.     VerbosePrinter vp;
  91.     std::cout << "Should print minutes and seconds:" << std::endl;
  92.     pd.print(vp);
  93.  
  94.     SinglePrinter sp;
  95.     std::cout << "Should print total seconds:" << std::endl;
  96.     pd.print(sp);
  97.  
  98.     EmptyPrinter ep;
  99.     std::cout << "Should print nothing:" << std::endl;
  100.     pd.print(ep);
  101.  
  102.     return 0;
  103. }
Success #stdin #stdout 0s 15240KB
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stdin
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Standard input is empty
stdout
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Should print minutes and seconds:
3m
22.5s
Should print total seconds:
202.5s
Should print nothing:
nothing
https://ideone.com/C5MbPU
language:
C++ (gcc 8.3)
created:
6 years ago
visibility:
public

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