See subject for question: Is it legal to derive from a template
parameter?

I think I answered this question for somebody once (and the answer was
no if I remember correctly), but I just can't remember anymore.  If it
is illegal, what is the substitute and is a bug fix in order?

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---------------------
Siemel Naran


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<siemelnaran@my-deja.com> wrote in message
news:90hrl4$9ph$1@nnrp1.deja.com...
> See subject for question: Is it legal to derive from a template
> parameter?
>
> I think I answered this question for somebody once (and the answer was
> no if I remember correctly), but I just can't remember anymore.  If it
> is illegal, what is the substitute and is a bug fix in order?

Do you mean

template < class T >
class D : public T
{
...etc
};

then afaik, the answer is yes. I'm just revisiting a paper from the template
workshop at Erfurt which discusses this 'mixin' idiom and a solution to one
of it's problems. http://www.oonumerics.org/tmpw00/eisenecker.html

14.1/13 implies that the name is in scope and usuable at that point, so it's
known about.
14.6.1/3 says something similar and has the example
template < class T > class X : public Array<T> { /*...*/ };
14.6.2 has more of the same.

Ah, bingo!
14.6.2/4 - "If a base type is a dependent type"...and example
template < class T > struct Y : T { ... };

I'm convinced.

HTH

Ric


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siemelnaran@my-deja.com wrote:
>
> See subject for question: Is it legal to derive from a template
> parameter?
>
> I think I answered this question for somebody once (and the answer was
> no if I remember correctly), but I just can't remember anymore.  If it
> is illegal, what is the substitute and is a bug fix in order?

There are a couple of different things you might be referring to, which
as far as I can tell are legal:

template<class T> struct A : public T // Case 1: Legal
{
 struct B : public T { // Case 2: Legal
 } b;
};

A<int> a; // Illegal - can't derive from non-class type

struct C { };

A<C> ac; // Legal

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