how to specialize templated member functions of non-templated classes?

Question

suppose I have a file alpha.h:

class Alpha {
public:
    template<typename T> void foo();
};

template<> void Alpha::foo<int>() {}
template<> void Alpha::foo<float>() {}

If I include alpha.h in more than one cpp file and compile with GCC 4.4, it complains there are multiple definitions of foo<int> and foo<float> across multiple object files. Makes sense to me, so I change the last two lines to:

template<> extern void Alpha::foo<int>() {}
template<> extern void Alpha::foo<float>() {}

But then GCC says:

explicit template specialization cannot have a storage class

ok... so how am I supposed to do this correctly? I'm worried that C++ doesn't allow what I'm trying to do in the first place, in which case is there a good idiom that will accomplish the same thing?

Answer 1

use inline keyword

template<> inline void Alpha::foo<int>() {}

alternatively, provide implementation in separate cpp file

Answer 2

You can forward declare as well as the inline option:

// .h
template<> void Alpha::foo<int>();

//.cpp
template<> void Alpha::foo<int>() {}

Answer 3

From the ODR point of view, a fully (explicitly) specialized template is no longer a template, so it is subject to the same ODR principles as a non-template entity of the same kind. (There are some exceptions from that rule, I believe, but it is good enough for our purposes).

In your case, for ODR purposes a fully specialized function template is an ordinary function. So, as an ordinary function it should be declared in the header file and defined in one and only one implementation file.