explicit specialization of template class member function - c++

Question

I need to specialize template member function for some type (let's say double). It works fine while class X itself is not a template class, but when I make it template GCC starts giving compile-time 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?

Answer 1

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>) {
}
};

Answer 2

If one is able to used std::enable_if we could rely on SFINAE (substitution failure is not an error)
that would work like so (see LIVE):
#include <iostream>
#include <type_traits>
template <typename C> class X
{
public:
template <typename T,
std::enable_if_t<!std::is_same_v<double,T>, int> = 0>
void get_as() { std::cout << "get as T" << std::endl; }
template <typename T,
std::enable_if_t<std::is_same_v<double,T>, int> = 0>
void get_as() { std::cout << "get as double" << std::endl; }
};
int main() {
X<int> d;
d.get_as<double>();
return 0;
}
The ugly thing is that, with all these enable_if's only one specialization needs to be available for the compiler otherwise disambiguation error will arise. Thats why the default behaviour "get as T" needs also an enable if.

Answer 3

Probably the cleanest way to do this in C++17 and on-wards is to use a if constexpr in combination with the std::is_same_v type trait without explicitly specialisation at all:
#include <iostream>
#include <type_traits>
template <typename C>
class X {
public:
template <typename T>
void get_as() {
// Implementation part for all types
std::cout << "get as ";
// Implementation part for each type separately
if constexpr (std::is_same_v<double, T>) {
std::cout << "'double'";
} else if constexpr (std::is_same_v<int, T>) {
std::cout << "'int'";
} else {
std::cout << "(default)";
}
// Implementation part for all types
std::cout << std::endl;
return;
}
};
int main() {
X<int> d {};
d.get_as<double>(); // 'double'
d.get_as<int>(); // 'int'
d.get_as<float>(); // (default)
return EXIT_SUCCESS;
}
Try it here!
If you need to have a return type as well you could declare the return type as auto:
template <typename T>
auto get_as() {
if constexpr (std::is_same_v<double, T>) {
return 0.5;
} else {
return 0;
}
}