Question

In the common diamond scenario:

class MBase {
  public:
    MBase(int) {}
    virtual char const* vf() const = 0;
    virtual ~MBase() {}
};

class D1 : public MBase { //NOT VIRTUAL!!!
  public:
    D1() : MBase(1) {}
    char const* vf() const { return "D1"; }
};

class D2 : virtual public MBase {
  public:
    D2() : MBase(2) {}
    char const* vf() const { return "D2"; }
};

class Bottom : public D1, public D2 {
  public:
    char const* vf() const { return "Bottom"; }
}

What would change if I modified the inheritance modifier of Bottom to be virtual for both D1 and D2? I mean, would that make any impact:

class Bottom : public virtual D1, public virtual D2 {
  public:
    char const* vf() const { return "Bottom"; }
}

Answer 1

No it doesn’t. Each class chooses which of its direct base classes may be shared with others. Since Bottom is a leaf in your implementation and it has in both cases only one instance of D1 and D2, it will in both cases have exactly one nested object of these types. Since D1 refuses to share an MBase, Bottom will have two MBase objects nested in it as well.

However MBase is not directly accessible from both versions of Bottom since it is ambiguous. You would have to explicitly cast to D1 or D2 in order to access MBase through Bottom objects (precisely because there are two MBases).

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The difference would be if you had one D1 that virtually inherits from MBase and one that doesn’t.