一.sizeof模板化

#include 
    
     #include 
     
      template 
      
       class TypeSize {  public:    static size_t const value = sizeof(T);};int main(){    std::cout << "TypeSize
       
        ::value = "              << TypeSize
        
         ::value << std::endl;}
        
       
      
     
    

二.获取容器元素类型

 

#include 
    
     #include 
     
      #include 
      
       #include 
       
        #include 
        
         template 
         
           class ElementT; // primary templatetemplate 
          
           class ElementT
           
            
              > { // partial specialization public: typedef T Type;};template 
             
              class ElementT
              
               
                 > { // partial specialization public: typedef T Type;};template 
                
                 class ElementT
                 
                  
                    > { // partial specialization public: typedef T Type;};template 
                   
                    void print_element_type (T const & c){ std::cout << "Container of " << typeid(typename ElementT
                    
                     ::Type).name() << " elements.\n";}int main(){ std::stack
                     
                       s; print_element_type(s);}
                     
                    
                   
                  
                 
                
               
              
             
            
           
          
         
        
       
      
     
    

通过定义一个内部变量来简化上述工作

 

template 
    
      class ElementT { public:     typedef typename C::value_type Type; };
    

标准库都定义了这样的类型

std::cout << "Container of "    << typeid( std::stack
    
     ::value_type).name()    << " elements.\n";
    

通过此方法来简化模板参数数量.

原先原型:

template 
    
      T sum_of_elements (C const& c);
    

利用上面的内部type简化参数数量

template
    
      typename ElementT
     
      ::Type sum_of_elements (C const& c);
     
    

三.确定一个对象是否是类类型

参考:

#include 
    
     template
     
      class IsClassT{    typedef char _One;    typedef struct{
   char a[2];}_Two;    template
      
           static _One isClass(int T::* p);    template
       
            static _Two isClass(...);public:    enum { Yes = sizeof(isClass
        
         (NULL))==sizeof(_One) };    enum { No = !Yes };     };class MyClass { }; struct MyStruct { }; union MyUnion { }; void myfunc() { } enum{e1}e; // check by passing type as template argument template 
         
           void check() { if (IsClassT
          
           ::Yes) { std::cout << " IsClassT " << std::endl; } else { std::cout << " !IsClassT " << std::endl; } } template 
           
             class ElementT { public: typedef typename C::value_type Type; }; // check by passing type as function call argument template 
            
              void checkT (T) { check
             
              (); } int main() { std::cout << "int: "; check
              
               (); std::cout << "MyClass: "; check
               
                (); std::cout << "MyStruct:"; MyStruct s; checkT(s); std::cout << "MyUnion: "; check
                
                 (); std::cout << "enum: "; checkT(e); std::cout << "myfunc():"; checkT(myfunc); }
                
               
              
             
            
           
          
         
        
       
      
     
    

四.引用和限定符

incr 函数将会使apply模板函数演绎成T&& arg,需要注意这点

#include 
    
      template 
     
       void apply (T& arg, void (*func)(T)) {     func(arg); } void incr (int& a) {     ++a; } void print (int a) {     std::cout << a << std::endl; } int main() {     int x=7;     apply (x, print);     //error    //apply (x, incr); }
     
    

五.IfThenElse模板

用一个布尔值和特化技术来选择类型

template
    
      class IfThenElse; // partial specialization: true yields second argument template
     
       class IfThenElse
      
        { public:     typedef Ta ResultT; }; // partial specialization: false yields third argument template
       
         class IfThenElse
        
          { public:     typedef Tb ResultT; };