TITLE: uncaught_exception() considered flawed

(Newsgroups: comp.std.c++, 10 Nov 98)

DEROCCO: "Paul D. DeRocco" <pderocco@ix.netcom.com> wrote:
> > [code snipped]
> >
> > Nice, but it still has the problem, true of all things that use
> > uncaught_exception(), that it may be called while unwinding the stack on
> > behalf of some unrelated exception that it doesn't have anything to do
> > with, and be fooled into thinking it can't do something that it really
> > can, or should do something that it mustn't, etc.


ABRAHAMS: abrahams@motu.com

> With all due respect, it sounds like you are appealing to vague fears here.
> Please justify your claim with an example.


BONNARD: Valentin Bonnard <bonnardv@pratique.fr>

I agree that current uncaught_exception() specificatons are 
broken: this function indicates if there is an active exception 
somewhere in the system, not if we are currently doing the 
stack walkback:

struct E { ~E () { std::cout << std::uncaught_exception() << std::endl;
}

E x;
throw 0;

correctly prints true but

struct F { ~F () { E e; } } f;
throw 0;

incorrectly prints true ! (we aren't doing a walkback because 
of exceptions, we are normally exiting a function).

Note that

struct F { E e; /*~F () {} */ } f;
throw 0;

correctly prints true.

uncaught_exception() breaks basic functionnal decomposition; a 
function does its job, honors its exception spec, ect, it doesn't 
have to know that its caller is already playing with exceptions.

uncaught_exception() should indicate if we are destroyed because 
of a walk-back.