CODEWITHSUNDEEP
c++templatesiteratorsizetype-traits
axcelenator
axcelenator

Reputation: 1527

determine variable type by iterators c++

I have some generic function that is returning some variable of type <T>. I pass some 2 iterators to the function. Then, I try to calculate the size of the vector with the function distance. Inside the func I define a new type definition which is called v_size, but the <T> type of the vector is unknown. How can I calculate the size of the vector by passing only the iterators? 

template<class In, class T> 
         T my_func(In b, In e) 
{

 typedef typename vector<T>::size_type v_size;
 v_size size = distance(b, e); 
  ... 
  ...

}

Upvotes: 2

Views: 693

Answers (2)

IlBeldus
IlBeldus

Reputation: 1040

You can do 3 things: Deduct the type via the operator*() of the iterator: using v_size = vector<std::remove_reference<decltype(*b)>::type>::size_type;

or deduct the type using the return type of the distance function: using v_size = std::decltype(distance(b, e));

or, even better, remove the v_size altogether and let the compiler deduct the type: auto size = std::distance(b, e);

Upvotes: 1

songyuanyao
songyuanyao

Reputation: 172934

If you want to get the type pointed by the iterator, you can use std::iterator_traits. e.g.

template<class In> 
auto my_func(In b, In e) -> typename std::iterator_traits<In>::value_type
{
    using T = typename std::iterator_traits<In>::value_type;
    ...
}

BTW: I eliminated the 2nd template parameter T which only appears in the return type declaration and then can't be automatically deduced. Otherwise you have to specify it explicitly when calling the function.

Upvotes: 2

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