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| <title>Using the GNU Compiler Collection (GCC): Copy Assignment</title> |
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| <a name="Implicit-Copy_002dAssignment-for-Virtual-Bases"></a> |
| <h4 class="subsection">11.7.4 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: |
| </p> |
| <div class="smallexample"> |
| <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></div> |
| |
| <p>The C++ standard specifies that ‘<samp>Base::Base</samp>’ is only called once |
| when constructing or copy-constructing a Derived object. It is |
| unspecified whether ‘<samp>Base::operator=</samp>’ is called more than once when |
| the implicit copy-assignment for Derived objects is invoked (as it is |
| inside ‘<samp>func</samp>’ in the example). |
| </p> |
| <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>val</samp>’, ‘<samp>name</samp>’ (via |
| <code>strdup</code>), ‘<samp>bval</samp>’, and ‘<samp>name</samp>’ again. |
| </p> |
| <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. |
| </p> |
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