dereferencing null

[Chad J]

On 03/04/2012 09:43 PM, Adam D. Ruppe wrote:
> On Monday, 5 March 2012 at 02:32:12 UTC, Chad J wrote:
>> I hate hate HATE vague error messages that don't help me.
>
> In a lot of cases, getting more info is very, very easy:
>
> $ dmd -g -debug test9
> $ ./test9
> Segmentation fault
> $ gdb ./test9
> GNU gdb (GDB) 7.1
> [...]
> (gdb) r
> Starting program: /home/me/test9
> [Thread debugging using libthread_db enabled]
>
> Program received signal SIGSEGV, Segmentation fault.
> 0x08067a57 in _Dmain () at test9.d:12
> 12 bar.foo = 5;
> (gdb) where
> #0 0x08067a57 in _Dmain () at test9.d:12
> #1 0x0806eaf8 in _D2rt6dmain24mainUiPPaZi7runMainMFZv ()
> #2 0x0806e605 in _D2rt6dmain24mainUiPPaZi7tryExecMFMDFZvZv ()
> #3 0x0806eb3f in _D2rt6dmain24mainUiPPaZi6runAllMFZv ()
> #4 0x0806e605 in _D2rt6dmain24mainUiPPaZi7tryExecMFMDFZvZv ()
> #5 0x0806e5b4 in main ()
> (gdb) print bar
> $1 = (struct test9.Bar *) 0x0
>
>
>
> My gdb is out of the box unmodified; you don't need anything
> special to get basic info like this.
>

News to me.  I've had bad runs with that back in the day, but maybe 
things have improved a bit.

>
>
>
> There's two cases where null annoys me though:
>
> 1) if it is stored somewhere where it isn't supposed to be.
> Then, the location of the dereference doesn't help - the
> question is how it got there in the first place.
>

True, but that's a different problem space to me.  Non-nullable types 
would be really cool right about now.

> 2) Segfaults in the middle of a web app, where running it under
> the same conditions again in the debugger is a massive pain in
> the butt.
>

THIS.  This is why I expect what I expect.  It's not web apps in my 
case.  It's that I simply cannot expect users to run my code in a 
debugger.  That is just /not acceptable/.

>
> I've trained myself to use assert (or functions with assert
> in out contracts/invariants) a lot to counter these.

*quiver*
It's not that I don't like assertions, contracts, or invariants.  These 
are very cool.  The problem is that they don't help me when I missed a 
spot and didn't use assertions, contracts, or invariants.  Back to 
spending a bunch of time inserting writefln statements to do something 
that I should be able to accomplish with my eyeballs and a stack trace 
pretty much instantaneously.