Question

I have a Visual Studio 2008 C++ application where I am implementing a replacement for the standard allocator used in containers like std::vector. But, I’ve run in to an issue. My implementation relies on the allocator owning a handle to a resource. In the case where the rebind feature is used, I would need to transfer ownership of the handle to the new allocator. Something like this:

template< class T >
class MyAllocator
{
public:
    template< class U >
    explicit MyAllocator( const MyAllocator< U >& other ) throw() 
        :  h_( other.Detach() ) // can't do this to a `const`
    {
    };

    // ...

private:
    HANDLE Detach()
    {
        HANDLE h = h_;
        h_ = NULL;
        return h;
    };

    HANDLE h_;
}; // class MyAllocator

Unfortunately, I can’t relieve the old allocator of the handle ownership because it is const. If I remove const from the rebind constructor, then the containers won’t accept it.

error C2558: class 'MyAllocator<T>' : no copy constructor available or copy constructor is declared 'explicit'

Is there a good way around this issue?

Answer 1

What happens if you declare h_ as mutable?