Q: Template specialization with parameter pack - c++

Question

I'll get right to the question.
We have template specialization:
class TestClass
{
public:
template<typename T>
static T fn(const T& a);
}
// OK
template<typename T>
T TestClass::fn(const T& a)
{
// ... magic for any type here ...
}
// OK
template<>
int TestClass::fn(const int& a)
{
// ... magic for int type here ...
}
All okay. But what if I want to add a parameter pack to the function?
class TestClass
{
public:
template<typename T, typename... Args>
static T fn(const T& a, Args&& ...b);
}
// OK
template<typename T, typename... Args>
T TestClass::fn(const T& a, Args&& ...b)
{
// ... magic for any type here ...
}
// Error
template<typename... Args>
int TestClass::fn(const int& a, Args&& ...b)
{
// ... magic for int type here ...
}
Visual Studio gives error E0147.
How can I do this without adding a new function to the class?
Thanks in advance for your help!
Have a nice day!

Answer 1

In your first example
template<>
int TestClass::fn(const int& a)
{ /* ... */ }
you have a full specialization, that is permitted for C++ template functions/methods
But, given the declaration
template<typename T, typename... Args>
static T fn(const T& a, Args&& ...b);
your second example
template<typename... Args>
int TestClass::fn(const int& a, Args&& ...b)
{ /* ... */ }
become a partial specialization, that isn't permitted for C++ functions/methods.
But you can use overloading, avoiding specialization at all, adding a declaration for a different template method with the same name.
I mean
class TestClass
{
public:
template <typename T, typename ... Args>
static T fn (T const & a, Args && ... b);
template <typename ... Args>
static T fn (int const & a, Args && ... b);
};
template <typename T, typename ... Args>
T TestClass::fn (T const & a, Args && ... b)
{ /* ... */ }
template <typename ... Args>
int TestClass::fn (int const & a, Args && ... b)
{ /* ... */ }