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<h4 class="subsection">12.7.4 Implicit Copy-Assignment for Virtual Bases</h4>
<p>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:
<pre class="smallexample"> 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;
}
</pre>
<p>The C++ standard specifies that ‘<samp><span class="samp">Base::Base</span></samp>’ is only called once
when constructing or copy-constructing a Derived object. It is
unspecified whether ‘<samp><span class="samp">Base::operator=</span></samp>’ is called more than once when
the implicit copy-assignment for Derived objects is invoked (as it is
inside ‘<samp><span class="samp">func</span></samp>’ in the example).
<p>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 more than once. In the example, copying
proceeds in the following order: ‘<samp><span class="samp">val</span></samp>’, ‘<samp><span class="samp">name</span></samp>’ (via
<code>strdup</code>), ‘<samp><span class="samp">bval</span></samp>’, and ‘<samp><span class="samp">name</span></samp>’ again.
<p>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.
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