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  1. #include <iostream>
  2. #include <memory>
  3.  
  4. template<typename T>
  5. using require_scalar = typename std::enable_if<std::is_arithmetic<T>::value>::type;
  6.  
  7. template<typename T>
  8. using require_not_scalar = typename std::enable_if<not std::is_arithmetic<T>::value>::type;
  9.  
  10. class any_ptr
  11. {
  12.     struct WrapperBase
  13.     {
  14.         virtual ~WrapperBase(){}
  15.     };
  16.  
  17.     template< typename T >
  18.     struct Wrapper : WrapperBase, T
  19.     {
  20.         template<typename ... Args>
  21.         Wrapper( Args&&... args ):
  22.             T( std::forward<Args>(args)... ) {}
  23.     };
  24.  
  25.     template< typename T >
  26.     struct ScalarWrapper : WrapperBase
  27.     {
  28.     	T scalar;
  29.  
  30.     	template<typename ... Args>
  31.     	ScalarWrapper( Args&&... args ):
  32.     		scalar( std::forward<Args>(args)... ) {}
  33.     };
  34.  
  35.     std::unique_ptr<WrapperBase> mPtr;
  36.  
  37.     any_ptr( std::unique_ptr<WrapperBase> ptr ):
  38.         mPtr{ std::move(ptr) } {}
  39.  
  40.     template< typename T >
  41.     T* _access( std::false_type ) const
  42.     {
  43.         return dynamic_cast<T*>(mPtr.get());
  44.     }
  45.  
  46.     template< typename T >
  47.     T* _access( std::true_type ) const
  48.     {
  49.     	auto ptr = dynamic_cast<ScalarWrapper<T>*>(mPtr.get());
  50.         return ptr ? &ptr->scalar : nullptr ;
  51.     }
  52.  
  53.     template< typename T,
  54.               typename ... Args >
  55.     static any_ptr _make_any_ptr( std::false_type, Args&&... args )
  56.     {
  57.         return std::unique_ptr<WrapperBase>{ new Wrapper<T>{ std::forward<Args>(args)... } };
  58.     }
  59.  
  60.     template< typename T >
  61.     static any_ptr _make_any_ptr( std::true_type, T arg )
  62.     {
  63.         return std::unique_ptr<WrapperBase>{ new ScalarWrapper<T>{arg} };
  64.     }
  65.  
  66. public:
  67.  
  68. 	template< typename T >
  69. 	T* access()
  70. 	{ return _access<T>( typename std::is_arithmetic<T>::type{} ); }
  71.  
  72. 	template< typename T >
  73. 	T const* access() const
  74. 	{ return _access<T>( typename std::is_arithmetic<T>::type{} ); }
  75.  
  76.     template< typename T,
  77.               typename ... Args >
  78.     friend any_ptr make_any_ptr( Args&&... args )
  79.     {
  80.         return _make_any_ptr<T>( typename std::is_arithmetic<T>::type{}, std::forward<Args>(args)... );
  81.     }
  82. };
  83.  
  84. ////////////////////////////////////////////////////////////////////////////////
  85.  
  86. template <class T>
  87. void inspect(any_ptr& p, void (&fun)(T))
  88. {
  89.     using actual_t = typename std::remove_reference<T>::type;
  90.  
  91.     if( auto t = p.access<actual_t>() )
  92.         fun(*t);
  93. }
  94.  
  95. ////////////////////////////////////////////////////////////////////////////////
  96.  
  97. class printable
  98. {
  99. public:
  100.     virtual void print() const = 0;
  101. };
  102.  
  103. ////////////////////////////////////////////////////////////////////////////////
  104.  
  105. #include <string>
  106. using namespace std;
  107.  
  108. class foo : public printable
  109. {
  110. public:
  111.     foo(string const& s) : m_s(s) {}
  112.  
  113.     virtual void print() const
  114.     {
  115.         cout << "foo: " << m_s << "\n";
  116.     }
  117.  
  118. private:
  119.     string m_s;
  120. };
  121.  
  122. class bar : public printable
  123. {
  124. public:
  125.     bar(int i) : m_i(i) {}
  126.  
  127.     virtual void print() const
  128.     {
  129.         cout << "bar: " << m_i << "\n";
  130.     }
  131.  
  132. private:
  133.     int m_i;
  134. };
  135.  
  136. ////////////////////////////////////////////////////////////////////////////////
  137.  
  138. void print_printable(printable& p)
  139. {
  140.     p.print();
  141. }
  142.  
  143. ////////////////////////////////////////////////////////////////////////////////
  144.  
  145. int main()
  146. {
  147.     auto f = make_any_ptr<foo>("foo");
  148.     auto b = make_any_ptr<bar>(42);
  149.     auto c = make_any_ptr<int>(42);
  150.  
  151.     inspect(f, print_printable);
  152.     inspect(b, print_printable);
  153.  
  154.     auto ptr = c.access<int>();
  155.     std::cout << *ptr;
  156. }
  157.  
Success #stdin #stdout 0s 3432KB
comments (?)
stdin 
Standard input is empty
stdout 
foo: foo
bar: 42
42
https://ideone.com/4E9YZS
language:
C++14 (gcc 8.3)
created:
12 years ago
visibility:
public

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