15 Special member functions [special]

For an object with a non-trivial constructor, referring to any non-static member or base class of the object before the constructor begins execution results in undefined behavior. For an object with a non-trivial destructor, referring to any non-static member or base class of the object after the destructor finishes execution results in undefined behavior. [ Example:

struct X { int i; };
struct Y : X { Y(); };                  // non-trivial
struct A { int a; };
struct B : public A { int j; Y y; };    // non-trivial

extern B bobj;
B* pb = &bobj;                          // OK
int* p1 = &bobj.a;                      // undefined, refers to base class member
int* p2 = &bobj.y.i;                    // undefined, refers to member's member

A* pa = &bobj;                          // undefined, upcast to a base class type
B bobj;                                 // definition of bobj

extern X xobj;
int* p3 = &xobj.i;                      // OK, X is a trivial class
X xobj;

For another example,

struct W { int j; };
struct X : public virtual W { };
struct Y {
  int* p;
  X x;
  Y() : p(&x.j) {   // undefined, x is not yet constructed
    }
};

 — end example ]

To explicitly or implicitly convert a pointer (a glvalue) referring to an object of class X to a pointer (reference) to a direct or indirect base class B of X, the construction of X and the construction of all of its direct or indirect bases that directly or indirectly derive from B shall have started and the destruction of these classes shall not have completed, otherwise the conversion results in undefined behavior. To form a pointer to (or access the value of) a direct non-static member of an object obj, the construction of obj shall have started and its destruction shall not have completed, otherwise the computation of the pointer value (or accessing the member value) results in undefined behavior. [ Example:

struct A { };
struct B : virtual A { };
struct C : B { };
struct D : virtual A { D(A*); };
struct X { X(A*); };

struct E : C, D, X {
  E() : D(this),    // undefined: upcast from E* to A* might use path E* → D* → A*
                    // but D is not constructed

                    // “D((C*)this)” would be defined: E* → C* is defined because E() has started,
                    // and C* → A* is defined because C is fully constructed

  X(this) {}        // defined: upon construction of X, C/B/D/A sublattice is fully constructed
};

 — end example ]

Member functions, including virtual functions, can be called during construction or destruction ([class.base.init]). When a virtual function is called directly or indirectly from a constructor or from a destructor, including during the construction or destruction of the class's non-static data members, and the object to which the call applies is the object (call it x) under construction or destruction, the function called is the final overrider in the constructor's or destructor's class and not one overriding it in a more-derived class. If the virtual function call uses an explicit class member access and the object expression refers to the complete object of x or one of that object's base class subobjects but not x or one of its base class subobjects, the behavior is undefined. [ Example:

struct V {
  virtual void f();
  virtual void g();
};

struct A : virtual V {
  virtual void f();
};

struct B : virtual V {
  virtual void g();
  B(V*, A*);
};

struct D : A, B {
  virtual void f();
  virtual void g();
  D() : B((A*)this, this) { }
};

B::B(V* v, A* a) {
  f();              // calls V​::​f, not A​::​f
  g();              // calls B​::​g, not D​::​g
  v->g();           // v is base of B, the call is well-defined, calls B​::​g
  a->f();           // undefined behavior, a's type not a base of B
}

 — end example ]

The typeid operator can be used during construction or destruction ([class.base.init]). When typeid is used in a constructor (including the mem-initializer or default member initializer for a non-static data member) or in a destructor, or used in a function called (directly or indirectly) from a constructor or destructor, if the operand of typeid refers to the object under construction or destruction, typeid yields the std​::​type_­info object representing the constructor or destructor's class. If the operand of typeid refers to the object under construction or destruction and the static type of the operand is neither the constructor or destructor's class nor one of its bases, the behavior is undefined.

dynamic_­casts can be used during construction or destruction ([class.base.init]). When a dynamic_­cast is used in a constructor (including the mem-initializer or default member initializer for a non-static data member) or in a destructor, or used in a function called (directly or indirectly) from a constructor or destructor, if the operand of the dynamic_­cast refers to the object under construction or destruction, this object is considered to be a most derived object that has the type of the constructor or destructor's class. If the operand of the dynamic_­cast refers to the object under construction or destruction and the static type of the operand is not a pointer to or object of the constructor or destructor's own class or one of its bases, the dynamic_­cast results in undefined behavior. [ Example:

struct V {
  virtual void f();
};

struct A : virtual V { };

struct B : virtual V {
  B(V*, A*);
};

struct D : A, B {
  D() : B((A*)this, this) { }
};

B::B(V* v, A* a) {
  typeid(*this);        // type_­info for B
  typeid(*v);           // well-defined: *v has type V, a base of B yields type_­info for B
  typeid(*a);           // undefined behavior: type A not a base of B
  dynamic_cast<B*>(v);  // well-defined: v of type V*, V base of B results in B*
  dynamic_cast<B*>(a);  // undefined behavior, a has type A*, A not a base of B
}

 — end example ]