SHORT Re: Suggestion: "fix" assert(obj)

[Yoni Lavi]

Don Clugston Wrote:

> Walter Bright wrote:
> > Georg Wrede wrote:
> >> Walter Bright wrote:
> >>> Kristian Kilpi wrote:
> >>>
> >>>> The problem is that
> >>>>
> >>>>   assert(obj);
> >>>>
> >>>> does not first check if 'obj' is null.
> >>>
> >>>
> >>> Yes it does, it's just that the hardware does the check, and gives 
> >>> you a  seg fault exception if it is null.
> >>
> >> Asserts were INVENTED to *avoid segfaults*.
> > 
> > I don't know when assert first appeared. But I first encountered them in 
> > the 80's, when the most popular machine for programming was the x86. The 
> > x86 had no hardware protection. When you wrote through a NULL pointer, 
> > you scrambled the operating systems, and all kinds of terrible, 
> > unpredictable things ensued. Asserts were used a lot to try and head off 
> > these problems.
> > 
> > Enter the 286. What a godsend it was to develop in protected mode, when 
> > if you accessed a NULL pointer you got a seg fault instead of a 
> > scrambled system. Nirvana! What was even better, was the debugger would 
> > pop you right to where the problem was. It's not only asserts done in 
> > hardware, it's asserts with:
> > 
> > 1) zero code size cost
> > 2) zero runtime cost
> > 3) they're there for every pointer dereference
> > 4) they work with the debugger to let you know exactly where the problem is
> > 
> > Seg faults are not an evil thing, they're there to help you. In fact, 
> > I'll often *deliberately* code them in so the debugger will pop up when 
> > it hits them.
> 
> True, but forgetting to 'new' a class is an extremely common mistake. The first 
> time I ever used classes in D, I didn't 'new' it (I bet this will happen to 
> almost everyone from a C++ background!). Getting an AV with no line number is 
> pretty off-putting. This remains the #1 situation where I use a debugger. And I 
> hate using debuggers to find silly typos. Getting an assert failure with a line 
> number would be enormously more productive.
> 
> BTW, the same 'it segfaults anyway' argument could be used to some extent for 
> array bounds checking.
No, writes out of array bounds can overrun other variables of your program instead of segfaulting, so I don't think it's a good comparison

I whole-heartedly agree on the line number complaint, I can't imagine debugging not at source level in this day and age. Though it's always fun to see how well the optimizer handles 8-10 levels of inlining in template methods/classes in my C++ code :) I guess I'm spoiled by MSVC 8, which generates line number info even in release builds