在静态分配的对象上调用虚拟函数时是否使用虚拟表

Question

Here's an example:

struct A {
  virtual int f() const { return 1; }
};

struct B : A {
  virtual int f() const { return 2; }
};

int main() {
  B b;
  cout << b.f() << endl;
}

Is there going to be a virtual table lookup incurred to call f() on the instance of B , or not? Or is it possible, but compilers can optimize it out?

Answer 1

Depends on the compiler and optimization options.

But as a rule, when a modern compiler knows the most derived type of an object, it will optimize away (the indirection of) direct virtual calls on the objects.