C++ single template specialisation with multiple template parameters

Question

Hallo!

I would like to specialise only one of two template types. E.g. template <typename A, typename B> class X should have a special implementation for a single function X<float, sometype>::someFunc().

Sample code:

main.h:

#include <iostream>

template <typename F, typename I>
class B
{
public:
    void someFunc()
    {
        std::cout << "normal" << std::endl;
    };

    void someFuncNotSpecial()
    {
        std::cout << "normal" << std::endl;
    };
};

template <typename I>
void B<float, I>::someFunc();

main.cpp:

#include <iostream>
#include "main.h"

using namespace std;

template <typename I>
void B<float, I>::someFunc()
{
    cout << "special" << endl;
}

int main(int argc, char *argv[])
{
    B<int, int> b1;
    b1.someFunc();
    b1.someFuncNotSpecial();

    B<float, int> b2;
    b2.someFunc();
    b2.someFuncNotSpecial();
}

Compilation fails for class B. Is it true, that this is not possible in C++ in this way? What would be the best workaround?

[edit]

template <float, typename I> void B<float, I>::someFunc(); leads to main.h:26: error: ‘float’ is not a valid type for a template constant parameter

template <typename I> void B<float, I>::someFunc(); leads to main.h:27: error: invalid use of incomplete type ‘class B’

And I'm using gcc.

[edit]

I don't want to specialise the whole class, as there are other functions that don't have a specialisation.

Answer 1

You have to provide a partial specialization of the class template B:

template <typename I>
class B<float, I>
{
public:
    void someFunc();
};

template <typename I>
void B<float, I>::someFunc()
{
    ...
}

You can also just define someFunc inside the specialization.

However, if you only want to specialize a function, and not a class do e. g.

template <typename F, typename I>
void someFunc(F f, I i) { someFuncImpl::act(f, i); }

template <typename F, typename I>
struct someFuncImpl { static void act(F f, I i) { ... } };

// Partial specialization
template <typename I>
struct someFuncImpl<float, I> { static void act(float f, I i) { ... } };

But you can't specialize a function template without this trick.

Answer 2

Although you can totally specialize member functions of a class template, you cannot _partially specialize member functions. - Andrei Alexandrescu

Partial Class specialization is explained by the other posters.

You can, however, use overloading:

template <class T, class U> T fun(U obj); // primary template
template <class U> void Fun<void, U>(U obj); // illegal pertial
// specialization
template <class T> T fun (Window obj); // legal (overloading)

If you want to go deep into this, you can read about this issue in depth in "Modern C++ Design" by A. Alexandrescu.