Q: Template specialization with parameter pack

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)
 { /* ... */ }