Assert and the optional Message

[Jonathan M Davis]

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