how do you call this template constructor?

Question

Possible Duplicate:
C++ invoke explicit template constructor

Hi,

template <typename T>
class testing
{
public:

    template <typename R>
    testing()
    {
       // constructor A
    }

    template <typename R>
    testing(const R&)
    {
       // constructor B
    }
};

What is the syntax to invoke the constructor A?

I will need the type to be passed during the constructor call. Is there a way to call it? constructor B is a workaround as I only need to know the type not the whole object.

Thanks,

Stephen

Answer 1

You can't. The class template is based on the type T, so any template parameter you pass while instantiating the class will match T and not R.

Edit: You may also find this post useful: Constructor templates and explicit instantiation

You should go ahead with the workaround (constructor B). Most modern compilers will optimize out the unused parameter, so it should make no difference.

Answer 2

you can create workaround:

template<class A>
testing(boost::mpl::identity<A>);

// and instantiate like this
testing(boost::mpl::identity<A>());

I asked very similar question before http://stackoverflow.com/questions/2786946/c-invoke-explicit-template-constructor