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  1. #include <iostream>
  2. #include <string>
  3.  
  4. // Reusable
  5.  
  6. template<int i, class T>
  7. struct optional_type{
  8. 	enum{ pos = i};
  9. 	typedef T value_type;
  10.  
  11. 	T val_;
  12.  
  13. 	const T& value()const{return val_;}
  14. 	optional_type(T rv):val_(rv){}
  15.  
  16. 	optional_type():val_(){}
  17.  
  18.  
  19. };
  20.  
  21. template<int i, class T>
  22. struct required_type{
  23. 	enum{ pos = i};
  24. 	typedef T value_type;
  25.  
  26. 	T val_;
  27.  
  28. 	const T& value()const{return val_;}
  29. 	required_type(T rv):val_(rv){}
  30.  
  31. private:
  32. 	required_type();
  33.  
  34.  
  35. };
  36.  
  37. template<int i, class T,class F>
  38. struct defaulted_type{
  39. 	enum{ pos = i};
  40. 	typedef T value_type;
  41. 	F f;
  42. 	T val_;
  43.  
  44. 	const T& value()const{return val_;}
  45. 	defaulted_type(T rv):val_(rv){}
  46.  
  47. 	defaulted_type():val_(f()){}
  48.  
  49.  
  50. };
  51.  
  52.  
  53.  
  54.  
  55. template<class... Args>
  56. struct arg_holder:public Args...{
  57.  
  58. 	template<class T>
  59. 	const typename T::value_type& get()const{
  60. 		const T* ret = this;
  61. 		return ret ->value();
  62. 	}
  63.  
  64. 	template<class... T>
  65. 	arg_holder(T... t):T(t)...{};
  66.  
  67.  
  68. };
  69.  
  70. // What you need to implement for a function
  71. struct user_arg_default{
  72. 	std::string operator()(){return "Somebody";};
  73. };
  74.  
  75. typedef optional_type<0,int> value;
  76. typedef defaulted_type<1,std::string,user_arg_default> user;
  77. typedef required_type<2,int> required;
  78.  
  79.  
  80. void greet(const arg_holder<value,user,required>& a){
  81. 	std::cout << "Hello " << a.get<user>() << " the value is " << a.get<value>()
  82. 		<< " Required " << a.get<required>() <<  std::endl;
  83. }
  84.  
  85. int main(){
  86. 	greet({required(11)});
  87. 	greet({required(11),value(10)});
  88. 	greet({user("John"),required(11),value(10)});
  89. 	greet({required(11),user("John")});
  90.  
  91. 	// the required parameter is required, if you uncomment below, you will get an error
  92. 	//greet({});
  93. 	//greet({value(10)});
  94. 	//greet({user("John"),value(10)});
  95. 	//greet({user("John")});
  96. }
Success #stdin #stdout 0s 3032KB
comments (?)
stdin
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Standard input is empty
stdout
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Hello Somebody the value is 0 Required 11
Hello Somebody the value is 10 Required 11
Hello John the value is 10 Required 11
Hello John the value is 0 Required 11
https://ideone.com/StubBS
language:
C++14 (gcc 8.3)
created:
11 years ago
visibility:
public

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