Member data in class derived from template

[Matthias von Faber]

Hello,

The code below compiles fine on VS 6.0 and .NET 2005, but G++ complains
about mData not having been declared in the Derived class:

template<typename T>
class Base
{
public:
Base() {}
virtual ~Base() {}

protected:
int mData;
};
template<typename T>
class Derived : public Base<T>
{
public:
Derived() {}
virtual ~Derived() {}

void f()
{
mData = 0;
}
};
I have done extensive searches on the problem, including books etc., but I
have found nothing on the topic. So I am beginning to feel that I might be
fundamentally confusing something...

But then again, a Derived<T> should be a Base<T>, and those should come
with an mData member. So what could it be?

Thanks,
--
Matthias

[Pete Becker]

Matthias von Faber wrote:

But then again, a Derived<T> should be a Base<T>, and those should come
with an mData member. So what could it be?

But Base<int>, for example, can be specialized, and it isn't required to
have a member named mData. So you have to tell the compiler where mData
comes from. Instead of the unadorned

mData = 0;

you can say

this->mData = 0;
Base<T>::mData = 0;

and I think there's a third one, that isn't popping into my head at the
moment.

--

Pete Becker
Roundhouse Consulting, Ltd.

[Rolf Magnus]

Matthias von Faber wrote:

Hello,

The code below compiles fine on VS 6.0 and .NET 2005, but G++ complains
about mData not having been declared in the Derived class:

template<typename T>
class Base
{
public:
Base() {}
virtual ~Base() {}

protected:
int mData;
};
template<typename T>
class Derived : public Base<T>
{
public:
Derived() {}
virtual ~Derived() {}

void f()
{
mData = 0;
}
};
I have done extensive searches on the problem, including books etc., but I
have found nothing on the topic. So I am beginning to feel that I might be
fundamentally confusing something...

But then again, a Derived<T> should be a Base<T>, and those should come
with an mData member. So what could it be?

Two-phase name loopkup. The name has to be a dependant name. Since the Base
template is not instantiated at the point of Derived's definition, the
compiler doesn't know yet that mData is supposed to be a member of the
object.
Try:

void f()
{
this->mData = 0;
}

[Alf P. Steinbach]

* Pete Becker:

Matthias von Faber wrote:

But then again, a Derived<T> should be a Base<T>, and those should come
with an mData member. So what could it be?

But Base<int>, for example, can be specialized, and it isn't required to
have a member named mData. So you have to tell the compiler where mData
comes from. Instead of the unadorned

mData = 0;

you can say

this->mData = 0;
Base<T>::mData = 0;

and I think there's a third one, that isn't popping into my head at the
moment.

You're probably thinking of a using-declaration.
--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

[Matthias von Faber]

> this->mData = 0;

That fixed it nicely. Well, seems that I should read up on class templates
then. My conception was that all Base specializations would have an mData.

Thanks very much!
--
Matthias