Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!uw-beaver!mit-eddie!rutgers!att!ulysses!andante!alice!ark
From: ark@alice.UUCP (Andrew Koenig)
Newsgroups: comp.lang.c++
Subject: Re: Virtual functions in constructors
Message-ID: <9127@alice.UUCP>
Date: 1 Apr 89 00:21:30 GMT
References: <1129@internal.Apple.COM>
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 67

In article <1129@internal.Apple.COM>, arn@apple.com (Arn Schaeffer) writes:

> Option #1:
> Do not allow virtual function calls in constructors.

> Option #2:
> Because base classes are initialized before derived classes, a virtual 
> function appearing in the constructor of a base class should call the 
> virtual function as it was defined for the base class.

> Option #3:
> Virtual functions always invoke the definition in the derived class.

Option 2 is correct.

Imagine a base class B and a class D derived from B.  Constructing
a D object involves first constructing the B that is part of it.

If a virtual function in B were called by B::B(), and it were
to a member of D, that would be executing a member of D before
initializing the D object.

Because D::D() can control which particular constructor of B
gets called, it is possible to make B behave differently when
constructed as part of a D than when constructed independently.
It is even possible to protect it:

	class B {
	public:
		B();
	protected:
		B(int);
		// and so on
	};

	class D: public B {
	public:
		D(int n): (n) {
			/* stuff */
		}
		// and so on
	};

The (n) in the definition of D::D(int) is the argument to
be handed to the base class (B).  Because B::B(int) is
protected, it is impossible to create a B object directly
from an int (although bugs in some C++ implementations may
let it slip by anyway).


Incidentally, option 1 is practically impossible to enforce:

	class B:

	extern void fudge(B*);

	class B {
	public:
		B() { fudge(this); }
		// and so on
	};

If fudge() is separately compiled, how can the compiler know that
it should not call any virtual functions?
-- 
				--Andrew Koenig
				  ark@europa.att.com