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POWhy does virtual assignment behave differently than other virtual functions of the same signature?
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  1. POWhy does virtual assignment behave differently than other virtual functions of the same signature?
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    copied!<p>While playing with implementing a virtual assignment operator I have ended with a funny behavior. It is not a compiler glitch, since g++ 4.1, 4.3 and VS 2005 share the same behavior.</p> <p>Basically, the virtual operator= behaves differently than any other virtual function with respect to the code that is actually being executed.</p> <pre><code>struct Base { virtual Base&amp; f( Base const &amp; ) { std::cout &lt;&lt; "Base::f(Base const &amp;)" &lt;&lt; std::endl; return *this; } virtual Base&amp; operator=( Base const &amp; ) { std::cout &lt;&lt; "Base::operator=(Base const &amp;)" &lt;&lt; std::endl; return *this; } }; struct Derived : public Base { virtual Base&amp; f( Base const &amp; ) { std::cout &lt;&lt; "Derived::f(Base const &amp;)" &lt;&lt; std::endl; return *this; } virtual Base&amp; operator=( Base const &amp; ) { std::cout &lt;&lt; "Derived::operator=( Base const &amp; )" &lt;&lt; std::endl; return *this; } }; int main() { Derived a, b; a.f( b ); // [0] outputs: Derived::f(Base const &amp;) (expected result) a = b; // [1] outputs: Base::operator=(Base const &amp;) Base &amp; ba = a; Base &amp; bb = b; ba = bb; // [2] outputs: Derived::operator=(Base const &amp;) Derived &amp; da = a; Derived &amp; db = b; da = db; // [3] outputs: Base::operator=(Base const &amp;) ba = da; // [4] outputs: Derived::operator=(Base const &amp;) da = ba; // [5] outputs: Derived::operator=(Base const &amp;) } </code></pre> <p>The effect is that the virtual operator= has a different behavior than any other virtual function with the same signature ([0] compared to [1]), by calling the Base version of the operator when called through real Derived objects ([1]) or Derived references ([3]) while it does perform as a regular virtual function when called through Base references ([2]), or when either the lvalue or rvalue are Base references and the other a Derived reference ([4],[5]).</p> <p>Is there any sensible explanation to this odd behavior?</p>
 

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