Nathan Nathan - 3 months ago 16
C++ Question

Specialization of templated member function in templated class

I have a templated class with an templated member function

template<class T>
class A {
public:
template<class CT>
CT function();
};


Now I want to specialize the templated member function in 2 ways. First for having the same type as the class:

template<class T>
template<> // Line gcc gives an error for, see below
T A<T>::function<T>() {
return (T)0.0;
}


Second for type bool:

template<class T>
template<>
bool A<T>::function<bool>() {
return false;
}


Here is how I am trying to test it:

int main() {
A<double> a;
bool b = a.function<bool>();
double d = a.function<double>();
}


Now gcc gives me for the line marked above:

error: invalid explicit specialization before ‘>’ token
error: enclosing class templates are not explicitly specialize


So gcc is telling me, that I have to specialize A, if I want to specialize function, right?
I do not want to do that, I want the type of the outer class to be open ...

Is the final answer: it is not possible? Or is there a way?

Answer

Yes, this is the problem:

error: enclosing class templates are not explicitly specialized 

You cannot specialize a member without also specializing the class.

What you can do is put the code from function in a separate class and specialize that, much like basic_string depends on a separate char_traits class. Then then non-specialized function can call a helper in the traits class.

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