Re: typedef for template

From:

=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <daniel.kruegler@googlemail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Wed, 27 Jul 2011 17:05:15 CST

Message-ID:

<j0psjk$pbt$1@dont-email.me>

Am 27.07.2011 01:50, schrieb Venkat S:

On Jul 26, 5:34 am, Daniel Kr?gler<daniel.krueg...@googlemail.com>
wrote:

Am 25.07.2011 23:57, schrieb Venkat S:

Hi,
  I've searched and not found any reference for this, so I thought
it's ok to post.

template<typename T> class Foo { ... };

template<typename X> class Bar {

   typedef Foo<X> Foo; // is this a legal c++ construct?

   Foo fooObj;

};

Thanks
Venkat
PS: Had gone through s14 (Working Draft, 2011-02-28 N3242==11-0012),
but couldn't locate any formal description.

It seems to me that this example should be covered by 3.3.7
[basic.scope.class] p1 and should be ill-formed because the name 'Foo'
does not refer to exactly the same declaration. The most specific
standardeze seems to be that of bullet (2):

"A name N used in a class S shall refer to the same declaration in its
context and when re-evaluated in the completed scope of S. No diagnostic
is required for a violation of this rule."

One comparable example is this one:

typedef int I;

class D {
typedef I I; // error, even though no reordering involved
};

Thanks for your response.

There are two diffs I'm seeing from the example.
1. No scoping issue - as I've not overridden any prior definition.

You have changed the meaning of the name "Foo" from the original one (before the typedef declaration) where it referred to the class template Foo.

2. Syntactically, and "name"-wise I see Foo<X> different from Foo.
This doesn't matter?

It doesn't matter, because "Foo" is a name referring to the class template Foo before the typedef declaration and after the typedef declaration "Foo" refers to the typedef declaration (Thanks to Johannes Schaub for this detail).

I wonder what's a name in c++?.

This was recently clarified by accepting the resolution of core issue

http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#485

HTH & Greetings from Bremen,

- Daniel Kr?gler

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