Multiple (diamond) inheritance compiles without “virtual”, but doesn't with

Question

Given the following code (without virtual inheritance) :

class A
{
public:
    virtual void f() = 0;
};

class B : public A
{
 public:
    virtual void f()          


        

Answer 1

Your first scenario hierarchy corresponds to:

    F()   F()
     A     A
     |     |
 F() B     C F()
      \   /
        D 

Where D is not abstract, because there are two A subobjects in an object of type D: One that is made concrete by B through the lattice of B, and another that is made concrete through the lattice of C.

Unless you try to invoke the function F() on object of D there will not be any ambiguity.

Your second scenario hierarchy corresponds to:

       F()  
        A
      /   \
 F() B     C F()
      \   /
        D  

In this scenario, the object D has a single Base class A sub object, and it must override and provide implementation of the pure virtual function in that subobject.

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Herb Sutter's articles in Guru Of The Week(GOTW) are a nice read for Multiple Inheritance:

1. Multiple Inheritance Part I
2. Multiple Inheritance Part II
3. Multiple Inheritance Part III