更新于：16/8/28

//针对左值的情况.template
    
     constexpr T&& my_forward(typename std::remove_reference
     
      ::type& value)noexcept{	std::cout << "my-forward: &" << std::endl;		return (static_cast
      
       (value));}//针对右值的情况.template
       
        constexpr T&& my_forward(typename std::remove_reference
        
         ::type&& value)noexcept{	std::cout << "my-forward: &&" << std::endl;	return (static_cast
         
          (value));}
         
        
       
      
     
    

std::forward<T>用来完美转发，也就是说如果变量是个右值经过std::forward仍然是个右值，如果是个左值经过std::forward转发仍然是个左值.

demo:

#include 
    
     #include 
     
      #include 
      
       #include 
       
        template
        
          inlineconstexpr _Ty&& my_forward(	typename std::remove_reference<_Ty>::type& _Arg) _NOEXCEPT{	// forward an lvalue as either an lvalue or an rvalue	std::cout << "forward: &" << std::endl;	return (static_cast<_Ty&&>(_Arg));}template
         
           inlineconstexpr _Ty&& my_forward( typename std::remove_reference<_Ty>::type&& _Arg) _NOEXCEPT{ // forward an rvalue as an rvalue static_assert(!std::is_lvalue_reference<_Ty>::value, "bad forward call"); std::cout << "forward: &&" << std::endl; return (static_cast<_Ty&&>(_Arg));}template
          
           class Node {private: T value;public: ~Node() = default; Node(T&& value_) :value(std::move(value_)) //注意这里我用了move!因为我们还有一个构造函数是const T& { //std::cout << std::boolalpha // << typename std::is_same
           
            (value_))>::value // << typename std::is_same
            
             ::value // << typename std::is_same
             
              (value_))>::value // << std::endl; std::cout << "constructor: &&" << std::endl; } Node(const T& value_) :value(value_) { std::cout << "constructor: const&" << std::endl; } void print()const { std::cout << this->value << std::endl; }};template
              
               void wrapperFunction(T&& value){ print(value);}template
               
                void wrapFunction(T&& value){ print(std::forward
                
                 (value));}template
                 
                  void print(const T& value){ std::cout << "T& : " << value << std::endl;}template
                  
                   void print(T&& value){ std::cout << "T&& :" << value << std::endl;}template
                   
                    void packagePrint(T&& val){ using type = typename std::remove_reference
                    
                     
                      ::type>::type; Node
                      
                       (std::forward
                       
                        (val));}template
                        
                         void packagePrint(Type&& arg, Types&&... args){ using type = typename std::remove_reference
                         
                          
                           ::type>::type; Node
                           
                            (std::forward
                            
                             (arg)); packagePrint(std::forward
                             
                              (args)...);}int main(){ //case 1: int number = 50; //wrapperFunction(number); //output: T& : 50; //wrapperFunction(50); //output: T& : 50; //case 2: //wrapFunction(number); //output: T& : 50; //wrapFunction(50); //output: T&& :50; //case 3: packagePrint(20, number, std::basic_string
                              
                               ("shihuawoaini")); //注意这里!!!!!!!!!!!! int&& x = 10l; //此时的x其实是一个右值引用的左值!!!!!所以我们必须用std::forward才能保持. int& y = x; //int&& z = x; //error!. return 0;}