Using-declaration of an existing namespace type vs creating a type alias

Question

This is not a question about the difference between using and typedef for creating type aliases. I would like to provide access to an existing type from a namespace inside a code block or a function.

I found two different ways :

I can "include" the type with a using declaration :

using typename mynamespace::mytype;

Or I can create a type alias :

typedef mynamespace::mytype mytype;
using mytype = mynamespace::mytype; //C++11

Is there any difference ?
What are the pros and cons of each syntax ?
Which one is the most used/recommended ?

Thank you.

Related question : Using-declaration of an existing type from base class vs creating a type alias inside child class

StoryTeller - Unslander Monica · Accepted Answer · 2019-10-18T10:09:37.360

5

Is there any difference ?

A type alias for a name in a namespace can appear in a class

struct S { using mytype = mynamespace::mytype; };

while a using-declaration may not.

What are the pros and cons of each syntax ?

The previous point is a pretty big con if you are dealing with class scope.

Other than that the two approaches are pretty much similar. An alias is a new name that stands exactly for the type that is aliased. While a using declaration brings the existing name of the type into scope. If you use mytype for both, you won't notice a difference.

Which one is the most used/recommended ?

I doubt there's consensus on this. Use the one you have to when you have to (class scope), but stick to your team's style guide otherwise.

edited Oct 18 '19 at 10:09

answered Oct 18 '19 at 09:53

StoryTeller - Unslander Monica

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```struct test : base { using typename base::mytype; };``` can also be used. – Baptistou Oct 18 '19 at 10:08
@Baptistou - Your question was about namespaces, I answer in accordance to that. – StoryTeller - Unslander Monica Oct 18 '19 at 10:09
@Baptistou - Don't. Editing and invalidating existing answers is poor netiquette on SO. You can always ask another – StoryTeller - Unslander Monica Oct 18 '19 at 10:12
Ok I will do that and link to this post. – Baptistou Oct 18 '19 at 10:18
@Baptistou - Good call! That's how one creates a useful library of questions :) – StoryTeller - Unslander Monica Oct 18 '19 at 10:18

score 0 · Answer 2 · answered Oct 22 '19 at 12:02

I discovered another difference between the two syntaxes : it is not possible to define a type alias with the same name as an existing namespace in the same scope.

namespace name1 {
    struct name2 {};
}

namespace name2 {
    struct name3 {};
}

//error: 'typedef struct name1::name2 name2' redeclared as different kind of symbol
/* typedef name1::name2 name2; */

//OK
using typename name1::name2;

//error: 'name2' does not name a type
/* name2 val1 = {}; */

//OK disambiguation with keyword "struct"
struct name2 val2 = {};

//OK namespace qualifier with "::"
name2::name3 val3 = {};

int main(){
    //OK different scope
    typedef name1::name2 name2;
    name2 val = {};
}

Using-declaration of an existing namespace type vs creating a type alias

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