A workaround with templates for changing a type

Question

Considering this delegate class:

template <class DELEGATECLASS, class METHODPTRSPEC>
class CLDelegate
{
public:
    CLDelegate();
    CLDelegate( DELEGATECLASS* instancePtr, METHODPTRSPEC methodPtr );
    CLDelegate( const CLDelegate& rhs );

    DELEGATECLASS* pInstance_;
    METHODPTRSPEC pMethod_;
};

//-----------------------------------------------------------------------------
//
template <class DELEGATECLASS, class METHODPTRSPEC>
CLDelegate<DELEGATECLASS, METHODPTRSPEC>::CLDelegate()
    : pInstance_( nullptr )
    , pMethod_( nullptr )
{}

//-----------------------------------------------------------------------------
//
template <class DELEGATECLASS, class METHODPTRSPEC>
CLDelegate<DELEGATECLASS, METHODPTRSPEC>::CLDelegate( DELEGATECLASS*
        instancePtr, METHODPTRSPEC methodPtr )
    : pInstance_( instancePtr )
    , pMethod_( methodPtr )
{}

//-----------------------------------------------------------------------------
//
template <class DELEGATECLASS, class METHODPTRSPEC>
CLDelegate<DELEGATECLASS, METHODPTRSPEC>::CLDelegate( const CLDelegate& rhs )
    : pInstance_( rhs.pInstance_ )
    , pMethod_( rhs.pMethod_ )
{}

I would like to use this delegate within another class, but I would have to have template parameters for the whole class, which I do not want, like this:

template<class DELEGATECLASS, class METHODPTR>
class SomeClass
{
public:
    IOVFUCam();
    ~IOVFUCam();

    // Data Members
private:
    ...
    const CLDelegate<DELEGATECLASS, METHODPTR>& del_;
};

Since I do not want to use template parameters on the whole class I wonder if it is possible to have a dummy delegate in my class and change its type (at compile time ?) :

struct EmptyType
{};

class SomeClass
{
public:
    template<class DELEGATECLASS, class METHODPTR>
    IOVFUCam( CLDelegate<DELEGATECLASS, METHODPTR> del )
     : del_(del)
    {};

    ~IOVFUCam();

    // Data Members
private:
    ...
    const CLDelegate<EmptyType, EmptyType>& del_;
};


FrameHandler handler;
CLDelegate<FrameHandler,
           void(FrameHandler::*)( IOBuffer::Ptr )>
           del( &handler, &FrameHandler::ProcessImage );
SomeClass obj( del );

I rapidly read something that Alexandrescu wrote about TypeTraits, and I wonder if this could be applied to my issue, or another technique ?

Thanks a lot !

Answer 1

Have you seen std::allocator? It has a 'rebind' type that allows an std::allocator to give you an std::allocator at compile time.

template <class DELEGATECLASS, class METHODPTRSPEC>
class CLDelegate
{
public:
    template<class D, class M>
    struct rebind {
        typedef CLDelegate<D, M> type;
    };

    CLDelegate();
    CLDelegate( DELEGATECLASS* instancePtr, METHODPTRSPEC methodPtr );
    CLDelegate( const CLDelegate& rhs );

    DELEGATECLASS* pInstance_;
    METHODPTRSPEC pMethod_;
};

This way if you have a CLDelegate type already you can use the following code to get a different CLDelegate type.

typename MyDelType::rebind<NewDel, NewSpec>::type

Is that what you were looking for?