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

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 开发者_JS百科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?

use inline keyword

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

alternatively, provide implementation in separate cpp file

You can forward declare as well as the inline option:

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

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

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.

No separate declaration or inline keywork is required. PF below working code.

#include<iostream>

class temp
{
public:
    template <class T>
    T add1(T a, T b)
    {
            return (a + b);
    }
};

template<>
std::string temp::add1<std::string>(std::string aa, std::string bb)
{
    return aa+bb;
}

int main()
{
    temp *tc = new temp();
    std::cout << tc->add1<float>(5.7, 4.5) << std::endl;
    std::cout << tc->add1<std::string>("my ","program") << std::endl;
}

output is :

10.2

my program