Hi,

I try to understand the decoupled design based on interfaces but I see that some problems arise and I have not found a place where this is discussed. The client code works with interface classes, decoupled from their implementation:

class IItem
{
public:
    virtual ~IItem() { }
    virtual bool get_name(string& name) = 0;                     // some
common properties
    virtual bool get_size(unsigned long &size) = 0;
}

class IAItem : public IItem
{
  public:
        // methods specific to IAItem
}

class IBItem : public IItem
{
  public:
        // methods specific to IBItem
}


// client code uses
IItem* pItem = ...
string name;
pItem->get_name(name);
usigned long size;
pItem->get_size(size);


The implementation will want to have a common abstract class derived from IItem where these common properties are implemented:

class ItemAbstract : public IItem
{
 public:
    virtual bool get_name(string& name) { ... }                    // some
common properties implemented
    virtual bool get_size(unsigned long &size) { .... }
}

but the problem appears with the implementation of the IAItem or IBItem:

class AItem : public IAItem, public ItemAbstract
{
public:
    // implementation of IAItem
}



                          IItem
           IAItem              ItemAbstract
                         AItem

this implementation is ambigous because the AItem class will have 2 instances of IItem (one inherited through IAItem and one through the ItemAbstract) so i need that:

class IAItem : virtual public IItem
class ItemAbstract : virtual public IItem

But, although this works, overseeing (for the moment) that fact that this
will give an warning saying that AItem inherits the ItemAbstract methods
through dominance, the problem i see is that at one moment I needed to
change the initial design of the interface.
What if I could not do that, i.e. maybe those interfaces are from a library
or something, hence read-only for me (the implementor) ?
It feels like the design is not OK but in the same way, since what's it's
trying to achieve is so simple, I can't see another way.

What's your opinion ?

Thanks.