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    <p>The abstract base class cannot help in case of assignment because the base sub-object is not instantiated (what an abstract class would block) but is sliced off the derived object (i.e. the assignment is done between already existing base sub-objects).</p> <p>To avoid the problem the only solution I can think to is</p> <ol> <li>make the assignment virtual</li> <li>check in the assignment that the source instance is of the correct type</li> </ol> <p>In code</p> <pre><code>#include &lt;iostream&gt; struct Base { int bx; Base(int bx) : bx(bx) {} virtual Base&amp; operator=(const Base&amp; other) { bx = other.bx; return *this; } }; struct A : Base { int x; A(int bx, int x) : Base(bx), x(x) {} A&amp; operator=(const Base&amp; other) { const A&amp; other_a = dynamic_cast&lt;const A&amp;&gt;(other); Base::operator=(other); x = other_a.x; return *this; } }; struct B : Base { int x; B(int bx, int x) : Base(bx), x(x) {} B&amp; operator=(const Base&amp; other) { const B&amp; other_b = dynamic_cast&lt;const B&amp;&gt;(other); Base::operator=(other); x = other_b.x; return *this; } }; </code></pre> <p>The <code>dynamic_cast&lt;const A&amp;&gt;(other)</code> is the operation that will fail if the object passed to the assignment operator is not of the correct derived type (it can be a sub-derived object, but this should be logically ok for an assignment <strong>source</strong>).</p> <p>As an example:</p> <pre><code>int main(int argc, const char *argv[]) { Base *pa1 = new A(1, 2); Base *pa2 = new A(3, 4); Base *pb1 = new B(5, 6); Base *pb2 = new B(7, 8); *pa1 = *pa2; std::cout &lt;&lt; pa1-&gt;bx &lt;&lt; "/" &lt;&lt; dynamic_cast&lt;A*&gt;(pa1)-&gt;x &lt;&lt; "\n"; *pb1 = *pb2; std::cout &lt;&lt; pb1-&gt;bx &lt;&lt; "/" &lt;&lt; dynamic_cast&lt;B*&gt;(pb1)-&gt;x &lt;&lt; "\n"; std::cout &lt;&lt; "Ok so far\n"; *pa1 = *pb1; // Runtime error here (bad cast) return 0; } </code></pre>
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    1. POHow does the abstract base class avoid the partial assignment?
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    1. CO"the base sub-object is not instantiated (what an abstract class would block) but is sliced off the derived object (i.e. the assignment is done between already existing base sub-objects)." Good point.
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