模板化成员 function 在模板化 class 中的特化

Question

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 1

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.

Answer 2

You can use overload, if you change the implementation.

template <typename T>
class Foo
{
public:
  template <typename CT>
  CT function() { return helper((CT*)0); }

private:
  template <typename CT>
  CT helper(CT*);

  T helper(T*) { return (T)0.0; }

  bool helper(bool*) { return false; }
};

Simple and easy:)