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c++c++11referencetype-traitsqualifiers
Vincent
Vincent

Reputation: 60481

std::remove_reference or std::remove_cv first?

If I want to extract the type of a const reference (like double from const double&), do I have to use :

typename std::remove_cv<typename std::remove_reference<Type>::type>::type

or

typename std::remove_reference<typename std::remove_cv<Type>::type>::type

?

Upvotes: 19

Views: 3694

Answers (2)

jrok
jrok

Reputation: 55425

Use remove_reference first. remove_cv removes top level qualifiers only and in case of references, there isn't any (or is ignored).

An example that shows the difference:

#include <iostream>
#include <type_traits>

template<typename T>
using Remove_cv_ref = std::remove_cv<typename std::remove_reference<T>::type>;

template<typename T>
using Remove_ref_cv = std::remove_reference<typename std::remove_cv<T>::type>;

int main()
{
    std::cout << std::is_same<typename Remove_cv_ref<const int&>::type, int>::value; // 1
    std::cout << std::is_same<typename Remove_ref_cv<const int&>::type, int>::value; // 0
}

Live demo.

Upvotes: 21

gx_
gx_

Reputation: 4770

typename std::remove_cv<typename std::remove_reference<Type>::type>::type

because first remove_reference<const double&>::type is const double, then remove_cv<const double>::type is double.

But if you have C++11, have a look at std::decay.

Upvotes: 5

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