c++ - Generic function object.

Question

I just saw this piece of code when I was looking at the stl source code analysis copy function

template<class InputIterator,class OutputIterator>
inline OutputIterator copy(InputIterator first,InputIterator last,OutputIterator result)
{
    return __copy_dispatch<InputIterator,OutputIterator>()(fist,last,result);//这是个函数
}

//这是完全泛化的的版本。
template <class InputIterator,class OutputIterator>
struct __copy_dispatch
{
    OutputIterator operator()(InputIterator first,InputIterator last,OutputIterator result)
    {
        return __copy(first,last,result,iterator_category(first));
    }    
};

This __copy_dispatch is a struct with an overloaded () operator. When called in copy, it directly

__copy_dispatch<InputIterator,OutputIterator>()(fist,last,result);

Directly The () operator is called using this struct, but is not called with a stuct object .

Is this okay? I quickly looked through c++primer and couldn't find the answer.
Please help me answer this question. thanks, thanks.

Answer 1

__copy_dispatch<InputIterator,OutputIterator>()

This is calling class

__copy_dispatch<InputIterator,OutputIterator>

The default constructor of

, its function is to generate a temporary object. Next

(fist,last,result)

The function of

is to call operator() on this temporary object with first, last, and result as actual parameters.

Answer 2

Using this struct, the () operator is called. This is the meaning of functor. Its function here is to create an unnamed object

Answer 3

In fact, lambda also created a class that overloads operator() to achieve the effect of functor.