Hi Axel;-)
>
>
> template <int N> class TBase {
> public:
> int Array[N];
> int fb1() { return Array[0]; };
> }; // everything is OK in TBase
>
> template <int NN> class TDerived : public TBase<NN> {
> int UU;
> public:
> int fc2() { return Array[0]; }; // error: 'Array' was not declared in this scope
> int fc3() { return UU; }; // OK
> int fc4() { return fb1(); }; // error: .... 'fb1' must be available
> };
>
> int main() {
> TDerived<10> Derived;
> Derived.Array[2] = 5; // OK
> Derived.UU = 1; // correct error, because UU is private
> Derived.fb1(); // OK
> return 0;
> }
>
> Why is it not possible to access the Members of the base template
> class TBase in the scope of the derived class TDerived ?
As I understood the C++-standard, the compiler is right: When the
compiler reads the definition of class TDerived, it does NOT know the
value of NN, so it does NOT know, whether TBase<NN> really has members
"Array" and "fb1". And it is possible (and absolutely legal C++-code) to
write a template specialization of TBase for some values of NN, which
does NOT have the members "Array" and "fb1".
So: You have to state explicitly the Base-class.
>
> I can fix this by writing:
> int fc2() { return TBase<NN>::Array[0]; };
> but in our production code, the base class is a template with 4
> parameters and its members are used a lots of times in the derived
> classes, so changing all these does not make the code very readable.
There is also another possibility -- "using":
If you add
using TBase<NN>::Array;
using TBase<NN>::fb1;
in the definition of TDerived, the compiler will find "Array[0]" and
"fb1()" from the Base-class:
template <int NN> class TDerived : public TBase<NN> {
using TBase<NN>::Array;
using TBase<NN>::fb1;
int UU;
public:
int fc2() { return Array[0]; }; // error: 'Array' was not declared in this scope
int fc3() { return UU; }; // OK
int fc4() { return fb1(); }; // error: .... 'fb1' must be available
};
That will work also. However, there is one difference between both
solutions: when you have a template specialization like "template <>
class TBase<5>{};" which does NOT have the members Array and fb1:
- my proposal with "using TBase..:" will NOT compile
- your code (writing TBase<NN>:: at every access) will compile, as long
as you dont call those functions (you could call however "fc3"
without problems)
HTH,
Axel
