How to forward declare a class which is in a namespace

To forward declare class type a in a namespace ns1:

namespace ns1
{
    class a;
}

To forward declare a type in multiple level of namespaces:

namespace ns1
{
  namespace ns2
  {
    //....
     namespace nsN
     {
        class a;
     }
    //....    
  }
}

Your are using a a member of consumer which means it needs concrete type, your forward declaration won’t work for this case.