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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& 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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