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Mirek Fidler
Guest
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 Posted: Mon Jul 14, 2003 5:41 pm Post subject: template class with template base class confusion... |
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template <class T>
struct Base {
T x;
};
template <class T>
struct Derived : public Base<T> {
Derived() { x = 10; }
};
does not compile with Comeau C++.
Anyway, by introducing dependant name
template <class T>
struct Derived : public Base<T> {
Derived() { Base<T>: = 10; }
};
everything works OK.
I am little bit confused about things standard says about this
situation. Can anybody explain please ?
Mirek
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Richard Smith
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| Posted: Tue Jul 15, 2003 11:50 pm Post subject: Re: template class with template base class confusion... |
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Mirek Fidler wrote:
| Quote: | template <class T
struct Base {
T x;
};
template
struct Derived : public Base
Derived() { x = 10; }
};
|
How does the compiler know what x is? Until the template is
instantiated for some particular T, specialisations of
Base<T> may be introduced, perhaps which do not have a
member called x. To resolve this problem, the compiler does
not look up x in the scope of Base<T> -- if a global
variable x exists, it will be used, otherwise the compiler
will issue a diagnostic.
There are two ways of forcing it to look up x in the scope
of the base class:
this->x = 10;
and
Base<T>: = 10;
I personally prefer the former syntax.
There's a very good explanation of this in Vandevoorde and
Josuttis excellent book "C++ Templates: The Complete Guide",
section 9.4.2. If you frequently write template code, I
strongly suggest you get a copy of this book.
--
Richard Smith
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Emil Dotchevski
Guest
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| Posted: Fri Jul 18, 2003 2:41 am Post subject: Re: template class with template base class confusion... |
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There is an even simpler example that does not compile under Comeau:
template <class T>
class Foo: T {
Foo(){x=0;}
};
"ComeauTest.c", line 4: error: identifier "x" is undefined
MSVC .NET reports no errors, but it is supposed to. The correct code is:
template <class T>
class Foo: T
{
Foo() { this->x=0; }
};
D&E goes into details why the standard is the way it is.
--Emil
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