I understand that the member names of a base-class template are hidden within the scope of a derived class, and therefore must be accessed using this->foo or Base<T>::foo. However, I recall that C++ also allows you to use the using keyword, which can come in handy in a derived-class function which frequently accesses a base-class variable. So, in order to avoid cluttering up the function with this-> everywhere, I'd like to use the using keyword.
I know I've done this before, but for whatever reason I can't get it to work now. I'm probably just doing something stupid, but the following code won't compile:
template <class T>
struct Base
{
int x;
};
template <class T>
struct Derived : public Base<T>
{
void dosomething()
{
using Base<T>::x; // gives compiler error
x = 0;
}
};
int main()
{
Derived<int> d;
}
The error, (with GCC 4.3) is: error: ‘Base<T>’ is not a namespace
Why doesn't this work?
It doesn't work because C++ language has no such feature and never had. A using-declaration for a class member must be a member declaration. This means that you can only use in class scope, but never in local scope. This all has absolutely nothing to do with templates.
In other words, you can place your using-declaration into class scope
struct Derived : public Base<T> {
...
using Base<T>::x;
...
};
but you can't have it inside a function.
Using-declarations for namespace members can be placed in local scope, but using-declarations for class members cannot be. This is why the error message complains about Base<T> not being a namespace.