6 Basics [basic]

#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
  ... = this;       // OK, this points to valid memory
}

void g() {
  void* p = std::malloc(sizeof(D1) + sizeof(D2));
  B* pb = new (p) D1;
  pb->mutate();
  *pb;              // OK: pb points to valid memory
  void* q = pb;     // OK: pb points to valid memory
  pb->f();          // undefined behavior: lifetime of *pb has ended
}

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

class T { };
struct B {
   ~B();
};

void h() {
   B b;
   new (&b) T;
}                               // undefined behavior at block exit

struct B {
  B();
  ~B();
};

const B b;

void h() {
  b.~B();
  new (const_cast<B*>(&b)) const B;     // undefined behavior
}