How to forward declare the following template class

I try to forward declare concurrent_bounded_queue ;

class MyClass {
    namespace tbb {
        template<typename T> class cache_aligned_allocator;
        template<class T, class A = cache_aligned_allocator> class concurrent_bounded_queue;
    };

    // I wish to maintain this syntax.
    tbb::concurrent_bounded_queue<std::string>& concurrentBoundedQueue;
}

---

I get the following error :

error C3203: 'cache_aligned_allocator' : unspecialized class template can't be used as a template argument for template parameter 'A', expected a real type
error C2955: 'tbb::cache_aligned_allocator' : use of class template requires template argument list c:\projects\vitroxreport\src开发者_如何转开发\Executor.h(21) : see declaration of 'tbb::cache_aligned_allocator'

May I know how I can avoid?

Thanks.

---

Allocator is a template, but second argument of the queue is concrete class. Try this:

class MyClass {
    namespace tbb {
        template<typename T> class cache_aligned_allocator;
        template<class T, class A = cache_aligned_allocator<T> > 
            class concurrent_bounded_queue;
    };

    tbb::concurrent_bounded_queue<std::string>& concurrentBoundedQueue;
};