Solve the diamond problem using virtual inheritance.
Virtual inheritance is the C++ mechanism designed specifically to solve the diamond problem that arises in multiple inheritance. When you specify that a base class is inherited `virtual`ly, you are telling the compiler that you only want a single instance of that base class's members to be included in any subsequent derived classes, no matter how many times it appears in the inheritance hierarchy. To make a base class virtual, you use the `virtual` keyword in the inheritance declaration of the intermediate classes. In our diamond problem example (`A` is the base, `B` and `C` inherit from `A`, and `D` inherits from `B` and `C`), you would declare the inheritance like this: `class B : virtual public A { ... };` and `class C : virtual public A { ... };`. Now, when class `D` inherits from both `B` and `C`, the compiler knows that `A` is a virtual base class and ensures that `D` contains only one copy of `A`'s members. The ambiguity is resolved, and you can access the members of `A` from an object of `D` without any issues. Virtual inheritance adds a small amount of overhead and complexity to object construction, so it should only be used when necessary to solve the diamond problem.