Why is "typename" necessary with the C++ "using XXX = type" type alias?

Asked Jul 27 '21 at 22:19

Active Jul 27 '21 at 22:19

Viewed 52 times

When declaring a type alias,

using T = XXX;

XXX can only be a type.

Why is it still necessary to include "typename" before XXX if XXX is a type alias internal to a templated class?

For example,

template <typename T> struct fred
{
    using T2 = T[2];
};

template <typename U> struct mary
{
    // why is typename necessary here?
    using M2 = typename fred <U>::T2;
    // this does not compile, even though type alias must refer to a type
    // using M2 = fred <U>::T2;
};

asked Jul 27 '21 at 22:19

Peter Fletcher

5

Did you read this? : [Where and why do I have to put the “template” and “typename” keywords?](https://stackoverflow.com/questions/610245/where-and-why-do-i-have-to-put-the-template-and-typename-keywords) – WhozCraig Jul 27 '21 at 22:22
With a type alias the dependent name can only be a type, so the "typename" keyword should not be necessary. – Peter Fletcher Jul 27 '21 at 22:25
4

Good news. C++20 refined the rules. Here's the proposal: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0634r3.html – StoryTeller - Unslander Monica Jul 27 '21 at 22:30
2

And you should read beyond just the accepted answer... [this answer](https://stackoverflow.com/a/67250787/817643) on the linked dupe discusses the proposal. – StoryTeller - Unslander Monica Jul 27 '21 at 22:32
Thanks craig and monica glad to see it's fixed in C++ 20 – Peter Fletcher Jul 27 '21 at 22:32
2

It's not "broken", hence "fixed" is inappropriate imo. Just a bit of cleaning up. – StoryTeller - Unslander Monica Jul 27 '21 at 22:34

Why is "typename" necessary with the C++ "using XXX = type" type alias?

0 Answers0