C ++中模板函数计数的语法

Question

I was following and example about function template Count and I am missing something. Surely I did not understand in full iterator_traits and i am confused:

Template Function COUNT has been declared as:

template <class InputIterator, class T>
  typename iterator_traits<InputIterator>::difference_type  
   count (InputIterator first, InputIterator last, const T& val);

My questions :

1) why using iterator_traits::difference_type ? The function returns an object why don't use a simple :

template <class InputIterator, class T>
 T count(InputIterator first, InputIterator last, const T& val); 

I have tried and worked.

2) in case i MUST use iterator_traits, why should I use a "different_type" (subtracting one iterator from another) and not "value_type" (the value the iterator can point to) ?

i have tried and worked even in this case.

I supposed iterator_traits should be used when a function returns an Iterator but then i have seen the template function FIND and it confused me more.

template <class InputIterator, class T>
 InputIterator find (InputIterator first, InputIterator last, const T& val);

whereas i would have coded like this:

template <class InputIterator, class T>
 typename iterator_traits<InputIterator>::value_type 
  find( nputIterator first, InputIterator last, const T& val);

Please can someone clarify this points ?

Thank you so much in advance for any help.

Answer 1

1) std::count is supposed to count things that are equal to something. So if your container holds, for example, apples, it would not be useful to return an apple object, because an apple is not useful for counting apples (at least not the kind of apples I have in mind)

std::vector<Apple> apples = ...;
Apple an_apple;
auto n = std::count(apples.begin(), apples.end(), an_apple);

Here, the type of n must be something that allows you to count a potentially large number of things. You are counting how many Apple objects are the same as an_apple .

2) std::find has to give you back something that can be used as a handle to an element of a container and to check whether the element exists or not. An iterator provides a good means to do exactly this. It would make no sense to return a value because a) you already have a copy of what you are looking for and b) you cannot generally return a "null" value to indicate that an element has not been found.

Answer 2

std::count应该返回某个元素的计数 ，因此返回类型必须是整数类型，但是T不一定是整数类型，因为它可以是任何东西（它是元素的类型，准确）。

Answer 3

1) The value the iterator points to can be anything, and not necessarily a value that can represent a count. count returns a numeric type, and the numeric type of choice is the difference_type of the iterator.

2) Again, value_type is the type of the thing the iterator points to, and not necessarily a numeric value.

Answer 4

Why would you want find() to return object you've just specified in find() function? What find does is looking for a place inside container where the object you're are looking is stored.