oops · high

The Diamond Problem & Virtual Inheritance

D inheriting B and C (both from A) yields two A subobjects and an ambiguous d.x; virtual inheritance makes A a single shared base.

🔑 Key line

Non-virtual diamond gives D two copies of A (ambiguous); virtual inheritance (B, C : virtual A) gives one shared A.

The code

struct A {
    int x;
};
struct B : A {}; // non-virtual
struct C : A {};
struct D : B, C {}; // TWO A subobjects -> d.x ambiguous

// Fix: virtual inheritance
struct B : virtual A {};
struct C : virtual A {};
struct D : B, C {}; // ONE shared A

What this lesson walks through

01The diamond
02Non-virtual: D gets TWO copies of A
03d.x is ambiguous
04Fix: make A a virtual base
05One shared A subobject
06The cost
07The rule

D derives from B and C, and both B and C derive from A. The inheritance graph forms a diamond with A at the top.

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