🚀 go-pugleaf

clamage@Eng.Sun.COM (Steve Clamage) writes:

>vandevod@cs.rpi.edu (David Vandevoorde) writes:
>>
>>SC> I think you would have to add this feature, or else disallow
>>SC> virtual calls from constructors and destructors.
>>
>>Right now, I think it makes sense disallowing them.
>
>The problem is actually a little more subtle. You also have to disallow
>passing the value of "this" out of a constructor or destructor, or 
>make the results of doing so undefined. A constructor (destructor) for T
>might call an ordinary global function which takes a pointer to T and
>calls a virtual function.

Even disallowing passing the value of `this' out of a ctor or dtor
would not be sufficient, because a ctor or dtor might call an ordinary
global function which gets a pointer which happens to be the same as
`this' by using the address of a global or static variable:

	struct Foo {
		Foo();
		virtual void virtual_func();
	}

	Foo x;

	void bar() {
		x.virtual_func();
	}

	Foo::Foo() {
		bar();
	}

On the other hand, disallowing passing the value of `this' out of
a ctor or dtor is not necessary either - you could simply say that
any direct *or indirect* call to a virtual function from within
a ctor or dtor gives undefined behaviour.  Mind you, I don't
think this would make things easier for programmers, and I wouldn't
advocate it.

--
Fergus Henderson             	WWW: http://www.cs.mu.oz.au/~fjh
fjh@cs.mu.oz.au              	PGP: finger fjh@128.250.37.3

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