How Virtual Dispatch Really Works (vtable indexing)

🔑 A virtual call is vptr + a compile-time-fixed slot index: (*obj->vptr[i])(obj). No name lookup.

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example.cpp

1struct Shape {
2    virtual double area();      // virtual #0  -> vtable slot [0]
3    virtual const char* name(); // virtual #1  -> vtable slot [1]
4    int id;                     // data member
5};
6struct Circle : Shape {
7    double area() override; // overrides slot [0]
8    double r;               // data member
9};
10 
11Shape* s = new Circle{}; // vptr set to Circle's vtable
12double a = s->area();    // (*s->vptr[0])(s)

Each virtual reserves a fixed slot index

At compile time the compiler walks Shape's virtual functions in declaration order and reserves a fixed slot: area() -> [0], name() -> [1]. These indices are baked into the binary and never change.

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Composition vs Inheritance (IS-A vs HAS-A)

Abstract Classes, Pure Virtual & Interfaces