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<h4 class="subsection">11.8.4 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&amp; operator= (const Base&amp; 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 &amp;d1, Derived &amp;d2)
     {
       d1 = d2;
     }
</pre>
 <p>The C++ standard specifies that &lsquo;<samp><span class="samp">Base::Base</span></samp>&rsquo; is only called once
when constructing or copy-constructing a Derived object.  It is
unspecified whether &lsquo;<samp><span class="samp">Base::operator=</span></samp>&rsquo; is called more than once when
the implicit copy-assignment for Derived objects is invoked (as it is
inside &lsquo;<samp><span class="samp">func</span></samp>&rsquo; in the example).

 <p>G++ implements the &ldquo;intuitive&rdquo; 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: &lsquo;<samp><span class="samp">val</span></samp>&rsquo;, &lsquo;<samp><span class="samp">name</span></samp>&rsquo; (via
<code>strdup</code>), &lsquo;<samp><span class="samp">bval</span></samp>&rsquo;, and &lsquo;<samp><span class="samp">name</span></samp>&rsquo; 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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