Is there another workaround for template base class inheritance?

Question

This code produces an error in the GNU compiler:

class A
{
public:
    int X;
};

template<class T>
class Foo : public T
{
public:
    void doStuff();
};

template<class T>
void Foo<T>::doStuff()
{
    X++;
}

There is already an answer for why this is an error. I would like to know if there is another way of working around this error rather than using

T::X

every time I want to reference the X member. I tried this:

template<class T>
void Foo<T>::doStuff()
{
    using T::X;
    X++;
}

But GCC gives an error: "a class-qualified name is not allowed"

obviously you cannot do `using T::X;` inside the function. You need to do this as shown here : http://www.ideone.com/MI9fM ... see my answer for other alternatives. They're not as good as this, but being aware of them helps sometimes! — Nawaz, Feb 11 '11 at 21:42

Nawaz · Accepted Answer · 2011-02-11T20:21:59.937

using T::X should work. If it gives error, there is some other problem with your code, or it must be compiler bug. See yourself : http://www.ideone.com/MI9fM

You may use this pointer, as
```
template<class T>
void Foo<T>::doStuff()
{
    this->X++;
}
```
This gives compiler a hint where to look for the symbol X.

Demo: http://www.ideone.com/NIucO

You can also do this:

template<class T>
void Foo<T>::doStuff()
{
     T::X++;
}

Demo : http://www.ideone.com/PswvG

UncleBens · Answer 2 · 2011-02-11T20:09:42.037

3

This would also work:

this->X++;

That would tell the compiler than since it sees X is not a member of the derived class, it has to come from the template base and there is no need to worry about it.

edited Feb 11 '11 at 20:09

answered Feb 11 '11 at 20:00

UncleBens

40,819
6
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Cheers and hth. - Alf · Answer 3 · 2011-02-11T20:48:08.050

1

class A
{
public:
    int X;
};

template<class T>
class Foo : public T
{
public:
    using T::X;
    void doStuff();
};

template<class T>
void Foo<T>::doStuff()
{
    X++;
}

int main()
{
    Foo<A> o;
    o.doStuff();
}

edited Feb 11 '11 at 20:48

answered Feb 11 '11 at 20:17

Cheers and hth. - Alf

142,714
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209
331

This is what I was missing, I put the using T::X in the doStuff() function instead of the class declaration. – camomilk Feb 11 '11 at 21:32

score 0 · Answer 4 · answered Feb 11 '11 at 21:08

0

Maybe this will work? Just guessing:

int &X = T::X;

This will create an alias that can be used within a block of code, similarly to the using T::X you tried that didn't work.

answered Feb 11 '11 at 21:08

Mark Ransom

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Is there another workaround for template base class inheritance?

4 Answers4