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Tyson Hilmer
Tyson Hilmer

Reputation: 771

How to get the value type of an iterator in a template declaration?

For this template declaration:

template <class Iterator>
typename std::iterator_traits<Iterator>::value_type Mean(Iterator begin, Iterator end);

I would like to simplify the value_type typename with a "using declaration" or typedef. So that it is a easier to read the return type. Is there a way to do this in the declaration?

Related posts 1 2

Upvotes: 0

Views: 100

Answers (2)

Botond Horv&#225;th
Botond Horv&#225;th

Reputation: 1017

You can create an alias to std::iterator_traits<Iterator>::value_type:

template <class It>
using itval= std::iterator_traits<It>::value_type;

//And use it:template <class Iterator>
itval<Iterator> Mean(Iterator begin, Iterator end);

In general for templates you should use the using keyword. Typedef can't support templates.

Upvotes: 0

NathanOliver
NathanOliver

Reputation: 180710

You can add an template alias like

template <typename Iterator>
using iterator_value_type_t = typename std::iterator_traits<Iterator>::value_type;

and then you can use that with your function like

template <class Iterator>
iterator_value_type_t<Iterator> Mean(Iterator begin, Iterator end);

If you want to allow specifying a custom return type then you can move the return type into the template parameter list like

template <class Iterator, 
          class ReturnType = typename std::iterator_traits<Iterator>::value_type>
ReturnType Mean(Iterator begin, Iterator end);

Upvotes: 2

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