function objects in C + + templates

In C + + template class map, a parameter is a compare type, which is a comparison function, which is fully defined as follows:

template<    class  Key,    class  T,    class Compare = Std::less <Key>,    class Allocator = std::allocator<std::p air<constclass map;

The function is passed as a class that uses the overloaded operator () to transform a function pointer to a class. In practice, you can also write your own class in this style.

  1  template <typename Key, TypeName Value, class  Hash > class   A {  3   has h Hash_fn;   public  :    A (): Hash_fn (hash ()) {  6   7    8   9  size_t hashcode (key key) {return HASH_FN (key);}  10  11 }; 
 

The third argument hash is a function object, which is implemented by the overloaded operator (), whose key of a string type is defined as follows:

1 struct Str_hash {2     operator () (const std::stringconst3    {4         return  0; 5     }6 };

When the HASH_FN (key) function is called, the overloaded function is called, and the conversion is implemented. Because the C + + standard library contains a large number of classes of this type of style, we can invoke the standard class by default parameters so that the template class A above can be defined as follows:

class Hash = std::hash<key>>

The test procedure is as follows:

1 intMainintargcChar*argv[]) {2     3A&LT;STD::string,int, str_hash>A;4Std::cout <<"hashcode:"<< A.hashcode ("ABC") <<Std::endl;5 6A&LT;STD::string,int, STD::HASH&LT;STD::string>>h;7Std::cout << H.hashcode ("ABC") <<Std::endl;8 9A&LT;STD::string,int>H2;TenStd::cout << H.hashcode ("ABC") <<Std::endl; One      A     return 0; -}

The test results are as follows:

1 Functiontem 2 hashcode:03335097746143350977461

function objects in C + + templates