Editing Virtual function (section)

== Behavior during construction and destruction ==
Languages differ in their behavior while the [[Constructor (computer science)|constructor]] or [[Destructor (computer science)|destructor]] of an object is running. For this reason, calling virtual functions in constructors is generally discouraged.

In C++, the "base" function is called. Specifically, the most derived function that is not more derived than the current constructor or destructor's class is called.<ref name="cpp-std">{{cite web|url=http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4659.pdf|title=N4659: Working Draft, Standard for Programming Language C++}}</ref>{{rp|§15.7.3}}<ref name=chen>{{cite web |title=What is __purecall? |last=Chen|first=Raymond|author-link=Raymond Chen|date=April 28, 2004 |url=https://devblogs.microsoft.com/oldnewthing/20040428-00/?p=39613}}</ref><ref>{{cite web |title=Never Call Virtual Functions during Construction or Destruction |last=Meyers|first=Scott|author-link=Scott Meyers|date=June 6, 2005 |url=http://www.artima.com/cppsource/nevercall.html}}</ref> If that function is a pure virtual function, then [[undefined behavior]] occurs.<ref name="cpp-std"/>{{rp|§13.4.6}}<ref name=chen /> This is true even if the class contains an implementation for that pure virtual function, since a call to a pure virtual function must be explicitly qualified.<ref>{{cite web |title=C++ corner case: You can implement pure virtual functions in the base class |last=Chen|first=Raymond|author-link=Raymond Chen|date=October 11, 2013 |url=https://devblogs.microsoft.com/oldnewthing/20131011-00/?p=2953}}</ref> A conforming C++ implementation is not required (and generally not able) to detect indirect calls to pure virtual functions at [[compile time]] or [[link time]]. Some [[runtime system]]s will issue a pure virtual function call error when encountering a call to a pure virtual function at [[Runtime (program lifecycle phase)|run time]].

In Java and C#, the derived implementation is called, but some fields are not yet initialized by the derived constructor (although they are initialized to their default zero values).<ref>{{cite web |title=Joy of Programming: Calling Virtual Functions from Constructors |last=Ganesh|first=S.G.|date=August 1, 2011 |url=https://www.opensourceforu.com/2011/08/joy-of-programming-calling-virtual-functions-from-constructors/}}</ref> Some [[Design pattern (computer science)|design patterns]], such as the [[Abstract Factory Pattern]], actively promote this usage in languages supporting this ability.