c++模板元编程2 - c++编程基础

polymorphic_downcast可以和简单的static_cast一样高效。使用该type traits设施来编写一个模板实现品，使其既可接收指针参数也可接收引用参数：
struct A { virtual ~A() {} };
struct B : A {};
B b;
A* a_ptr = &b;B* b_ptr = polymorphic_downcast (a_ptr);A& a_ref = b;
B& b_ref = polymorphic_downcast (a_ref);

这个题没有理解题意，不会做， ~~~~~~

2-3. 使用type traits
设施实现一个type_descriptor类模板，当被流化（streamed）时，其实例打印
其模板参数的类型：
std::cout< ()；//打印“char*”；
std::cout< ()；//打印“longconst*&”；
你可以假定type_descriptor的模板参数局限于
根据以下四种整型构建的复合类型：char、short int、int以及long int。

这个题比较简单，不说了直接贴代码：

template
        
         
struct get_description
{
	static std::string value;
	operator const char*()
	{
		return value.c_str();
	}
};

template
         
           std::string get_description
          
           ::value = "can not deduce the type"; template<> std::string get_description
           
            ::value = "int"; template<> std::string get_description
            
             ::value = "char"; template<> std::string get_description
             
              ::value = "short"; template<> std::string get_description
              
               ::value = "long"; template<> std::string get_description
               
                ::value = "float"; template<> std::string get_description
                
                 ::value = "double"; template
                 
                   struct get_description
                  
                    { operator const char*() { static std::string ret = get_description
                   
                    (); ret += " const"; return ret.c_str(); } }; template
                    
                      struct get_description
                     
                       { operator const char*() { static std::string ret = get_description
                      
                       (); ret += " *"; return ret.c_str(); } }; template
                       
                         struct get_description
                        
                          { operator const char* () { static std::string ret = get_description
                         
                          (); ret += " &"; return ret.c_str(); } };

测试代码：

void fun_type_descriptor()
{

	std::cout << get_description
        
         () << std::endl;
	std::cout << get_description
         
          ()<< std::endl; //std::cout << get_description
          
           ()<< std::endl; }

2-5. 修改练习2-3中的type_descriptor
模板，使其输出type的伪英语描述，就像cdecl程序的
explain命令所做的那样:
//打印“array of pointer to function returning pointer to char”
std::cout << type_descriptor< char *(*[])() >();

这个题也比较简单，直接上代码：

template
        
         
struct get_description
{
	static std::string value;
	operator const char*()
	{
		return value.c_str();
	}
};

template
         
           std::string get_description
          
           ::value = "can not deduce the type"; template<> std::string get_description
           
            ::value = "int"; template<> std::string get_description
            
             ::value = "char"; template<> std::string get_description
             
              ::value = "short"; template<> std::string get_description
              
               ::value = "long"; template<> std::string get_description
               
                ::value = "float"; template<> std::string get_description
                
                 ::value = "double"; template
                 
                   struct get_description
                  
                    { operator const char*() { static std::string ret = get_description
                   
                    (); ret += " const"; return ret.c_str(); } }; template
                    
                      struct get_description
                     
                       { operator const char*() { static std::string ret = get_description
                      
                       (); ret += " volatile"; return ret.c_str(); } }; template
                       
                         struct get_description
                        
                          { operator const char*() { static std::string ret = "pointer to "; ret += get_description
                         
                          (); return ret.c_str(); } }; template
                          
                            struct get_description
                           
                             { operator const char* () { static std::string ret = "reference to "; ret += get_description
                            
                             (); return ret.c_str(); } }; template
                             
                               struct get_description
                              
                                { operator const char* () { std::cout << typeid(T).name() << "\n"; static std::string ret = "array of "; ret += get_description
                               
                                (); std::cout << typeid(T).name() << "\n"; return ret.c_str(); } }; template
                                
                                  struct get_description
                                 
                                   { operator const char* () { std::cout << typeid(T).name() << "\n"; static std::string ret = "array of "; ret += get_description
                                  
                                   (); std::cout << typeid(T).name() << "\n"; return ret.c_str(); } }; template
                                   
                                     struct get_description
                                    
                                      { operator const char* () { static std::string ret = "pointer to function returning "; ret += get_description
                                     
                                      (); return ret.c_str(); } }; template
                                      
                                        struct get_description
                                       
                                         { operator const char* () { static std::string ret = "this is a pointer function wit

c++模板元编程2(二)