Re: typedef for template
On 26-Jul-11 6:04 PM, Daniel Kr?gler 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
};
Is there not a difference between a 'name' and a 'typedef-name'? I see section [dcl.typedef] being more relevant here. I am referring to N3242 so this might be different from current standard in this regard (if so, please correct me) and it says in [dcl.typedef]/p1:
"A name declared with the typedef specifier becomes a typedef-name."
If I look further [p3, p4, p5], it is not clear if the example you posted should be an error or not. That is,
1) In non-class scope example:
typedef struct s { / ... / } s;
typedef int I;
typedef int I;
typedef I I;
re-definition of I to refer to same type is considered to be OK.
2) In class scope example:
struct S {
typedef struct A { } A; // OK
typedef struct B B; // OK
typedef A A; // error
};
re-definition of same typedef-name is considered to be error here.
3) In any given scope:
class complex { / ... / };
typedef int complex; // error: redefinition
re-definition to some other type is an error, which is different case than the example since we are providing typedefs without changing the definition of the type.
Overall, looks like none of them represent the variation of your example. 1) comes pretty close, and if you consider that, it doesn't seem to be an error. Tried with Comeau Online compiler and it did not complain either but I guess that is not too conclusive.
Thanks and regards,
Abhishek Padmanabh
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