Re: anti-standard code can compile -- about template class

From:
"Igor Tandetnik" <itandetnik@mvps.org>
Newsgroups:
microsoft.public.vc.language
Date:
Fri, 7 Mar 2008 12:06:31 -0500
Message-ID:
<O#JKYWHgIHA.2268@TK2MSFTNGP02.phx.gbl>
George <George@discussions.microsoft.com> wrote:

I have tried if we try to instantise the code I posted in my original
question, it can not compile. Here is my code.


Then I guess the conformance has improved in later versions. I only have
VC7.1 handy at the moment to check, and your code compiles there.

Does this compile error
because of VC will look for global i other than base class i?


That's what a conforming compiler should do, yes.

I do
not know why if the class is instantised, still the global i other
than base class i is looked for?


Unqualified name that's not part of a member access expression (as in
something.name or something->name) is not considered a dependent name.
Thus, it is looked up when template definition is processed, not when
the template is instantiated (I'm assuming a compiler that properly
implements two-phase lookup; at least some MSVC versions don't). A side
effect of this is that the name is not looked up in the base class if
the base class is itself a template (the compiler cannot look inside a
template until its parameters are known, because there might be
specializations).

This is why in your example member variable Base<T>::i is not found by
name lookup.

template <typename T> struct Base {
public:
Base (int _i): i (_i)
{
}

int i;
};

template <typename T> struct Derived : public Base<T> {
public:
Derived (int _i) : Base<T> (_i)
{
}
int get_i() { return i; }
};


Make it

int get_i() { return this->i; }
// or
int get_i() { return Base<T>::i; }

The first variant is more common. Both turn 'i' into a dependent name,
thus delaying lookup until the template is instantiated.
--
With best wishes,
    Igor Tandetnik

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necessarily a good idea. It is hard to be sure where they are going to
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