Does the retention period of the new resource use the new object?

Question

#include <cstdlib>
struct B {
    virtual void f();
    void mutate();
    virtual ~B();
};
struct D1 : B { void f(); };
struct D2 : B { void f(); };
void B::mutate() {
    new (this) D2; // reuses storage — ends the lifetime of *this
    f(); // undefined behavior - WHY????
    ... = this; // OK, this points to valid memory
}

I need to explain why f()invokation has UB? new (this) D2;reuses storage, but also calls the constructor for D2, and also starts the lifetime of a new object. In this case, f()equal to this -> f(). That is, we just call f()the member function D2. Who knows why this is UB?

Answer 1

The standard shows this example § 3.8 67 N3690:

struct C {
  int i;
  void f();
  const C& operator=( const C& );
};

const C& C::operator=( const C& other) {
  if ( this != &other ) {
    this->~C(); // lifetime of *this ends
    new (this) C(other); // new object of type C created
    f(); // well-defined
  }
  return *this;
}

C c1;
C c2;
c1 = c2; // well-defined
c1.f(); // well-defined; c1 refers to a new object of type C

Note that this example completes the lifetime of an object before constructing a new object in place (compare with your code that does not call the destructor).

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Answer 2

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