template specialization for static member function

问题:

I am trying to implement a template function with handles void differently using template specialization.

The following code gives me an "Explicit specialization in non-namespace scope" in gcc:

template <typename T>
static T safeGuiCall(boost::function<T ()> _f)
{
    if (_f.empty())
        throw GuiException("Function pointer empty");
    {
        ThreadGuard g;
        T ret = _f();
        return ret;
    }
}

// template specialization for functions wit no return value
template <>
static void safeGuiCall<void>(boost::function<void ()> _f)
{
    if (_f.empty())
        throw GuiException("Function pointer empty");
    {
        ThreadGuard g;
        _f();
    }
}

I have tried moving it out of the class (the class is not templated) and into the namespace but then I get the error "Explicit specialization cannot have a storage class". I have read many discussions about this, but people don't seem to agree how to specialize function templates. Any ideas?

回答1:

When you specialize a templated method, you must do so outside of the class brackets:

template <typename X> struct Test {}; // to simulate type dependency

struct X // class declaration: only generic
{
   template <typename T>
   static void f( Test<T> );
};

// template definition:
template <typename T>
void X::f( Test<T> ) {
   std::cout << "generic" << std::endl;
}
template <>
inline void X::f<void>( Test<void> ) {
   std::cout << "specific" << std::endl;
}

int main()
{
   Test<int> ti;
   Test<void> tv;
   X::f( ti ); // prints 'generic'
   X::f( tv ); // prints 'specific'
}

When you take it outside of the class, you must remove the 'static' keyword. Static keyword outside of the class has a specific meaning different from what you probably want.

template <typename X> struct Test {}; // to simulate type dependency

template <typename T>
void f( Test<T> ) {
   std::cout << "generic" << std::endl;
}
template <>
void f<void>( Test<void> ) {
   std::cout << "specific" << std::endl;
}

int main()
{
   Test<int> ti;
   Test<void> tv;
   f( ti ); // prints 'generic'
   f( tv ); // prints 'specific'
}

回答2:

It's not directly an answer to your question but you can write this

template <typename T>
static T safeGuiCall(boost::function<T ()> _f)
{
        if (_f.empty())
                throw GuiException("Function pointer empty");
        {
                ThreadGuard g;
                return _f();
        }
}

It should work even if _f() return 'void'

Edit : In a more general case, I think we should prefer function overloading instead of specialization. Here is a good explanation for this : http://www.gotw.ca/publications/mill17.htm

回答3:

You can declare the explicit specialisation in the same way you'd define a member function outside of its class:

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

template <>
void A::foo<void> ()
{
}

回答4:

Your problem appears to be with boost::function - the following specialisations work:

template <typename T>
T safeGuiCall()
{
    return T();
}

template <>
void safeGuiCall<void>()
{
}

int main() {
    int x = safeGuiCall<int>();     // ok
    //int z = safeGuiCall<void>();  // this should & does fail
    safeGuiCall<void>();            // ok
}

回答5:

I had a similar problem. If you look at the original post, I left the first static in, but took out the second and BOTH errors went away.