GNU Info

(gcc-300.info)Copy Assignment

---

Prev: Temporaries Up: C++ Misunderstandings

---

Enter node , (file) or (file)node

Implicit Copy-Assignment for Virtual Bases
------------------------------------------

   When a base class is virtual, only one subobject of the base class
belongs to each full object.  Also, the constructors and destructors are
invoked only once, and called from the most-derived class.  However,
such objects behave unspecified when being assigned.  For example:

     struct Base{
       char *name;
       Base(char *n) : name(strdup(n)){}
       Base& operator= (const Base& other){
        free (name);
        name = strdup (other.name);
       }
     };
     
     struct A:virtual Base{
       int val;
       A():Base("A"){}
     };
     
     struct B:virtual Base{
       int bval;
       B():Base("B"){}
     };
     
     struct Derived:public A, public B{
       Derived():Base("Derived"){}
     };
     
     void func(Derived &d1, Derived &d2)
     {
       d1 = d2;
     }

   The C++ standard specifies that `Base::Base' is only called once
when constructing or copy-constructing a Derived object.  It is
unspecified whether `Base::operator=' is called more than once when the
implicit copy-assignment for Derived objects is invoked (as it is
inside `func' in the example).

   g++ implements the "intuitive" algorithm for copy-assignment: assign
all direct bases, then assign all members.  In that algorithm, the
virtual base subobject can be encountered many times.  In the example,
copying proceeds in the following order: `val', `name' (via `strdup'),
`bval', and `name' again.

   If application code relies on copy-assignment, a user-defined
copy-assignment operator removes any uncertainties.  With such an
operator, the application can define whether and how the virtual base
subobject is assigned.