I need to specialize a class template X's member function for some type (let's say double).
It works fine while class X itself is not a class template, but when I make it a template, GCC starts giving compiler errors.
#include <iostream>
#include <cmath>
template <class C> class X
{
public:
template <class T> void get_as();
};
template <class C>
void X<C>::get_as<double>()
{
}
int main()
{
X<int> x;
x.get_as();
}
Here is the error message:
source.cpp:11:27: error: template-id
'get_as<double>' in declaration of primary template
source.cpp:11:6: error: prototype for
'void X<C>::get_as()' does not match any in class 'X<C>'
source.cpp:7:35: error: candidate is:
template<class C> template<class T> void X::get_as()
What is the problem here, and how can I fix it?
It doesn't work that way. You would need to say the following, but it is not correct
template <class C> template<>
void X<C>::get_as<double>()
{
}
Explicitly specialized members need their surrounding class templates to be explicitly specialized as well. So you need to say the following, which would only specialize the member for X<int>.
template <> template<>
void X<int>::get_as<double>()
{
}
If you want to keep the surrounding template unspecialized, you have several choices. I prefer overloads
template <class C> class X
{
template<typename T> struct type { };
public:
template <class T> void get_as() {
get_as(type<T>());
}
private:
template<typename T> void get_as(type<T>) {
}
void get_as(type<double>) {
}
};