Assert and the optional Message

[Jacob Carlborg]

On 2012-03-09 12:12, Jonathan M Davis wrote:
> On Friday, March 09, 2012 11:03:38 Regan Heath wrote:
>> On Fri, 09 Mar 2012 08:20:33 -0000, Chris Pons<cmpons at gmail.com>  wrote:
>>> Any idea why, system("PAUSE") does work?
>>
>> As Jonathan says, assert throws AssertError... which means, if you wanted
>> to you could catch it, for example...
>>
>> module asserting;
>>
>> import std.stdio;
>> import core.exception;
>>
>> debug
>> {
>>       import std.c.windows.windows;
>>       extern (C) int kbhit();
>> }
>>
>> void main()
>> {
>>       try
>>       {
>>           assert(false, "testing assert");
>>       }
>>       catch(AssertError e)
>>       {
>>           debug
>>           {
>>               writefln("Assertion failed: %s", e);
>>               writefln("Press any key to exit...");
>>               while(!kbhit())
>>                   Sleep(1);
>>           }
>>           throw e;
>>       }
>> }
>>
>> So, for debug builds your exe will output the assertion manually, and wait
>> for a key press.  Then re-throw the assertion (resulting in the d runtime
>> outputting the assertion again).  In release, no pause.
>>
>> Note: kbhit and Sleep are windows specific (sleep would be the unix
>> equivalent, not sure on one for kbhit).
>
> Bet advised that catching anything not derived from Exception is generally a
> _bad_ idea (most of which are derived from Error), and AssertError is _not_
> derived from Exception but rather Error. AssertErrors are not guaranteed to
> invoke finally blocks, scope statements, or destructors. Your program is in an
> invalid state when you catch an Error. So, while it may work in this
> particular case, it is _not_ something that you should be doing in general. By
> far the worst thing to do would be to catch an Error and then continue
> executing. Errors are _supposed_ to kill your program.
>
> - Jonathan M Davis

Then I would say it's bad design to have "assert" throw an AssertError. 
Whatever "assert" throws needs to be catchable, this is essential for 
writing unit test frameworks. It needs to be possible to write a unit 
test framework that catches all AssertErrors and prints a nice report at 
the end (or similar).

-- 
/Jacob Carlborg