Re: Pre-condition vs. Post-condition

[jls@rutabaga.Rational.COM (Jim Showalter)]

>procedure pop(s: stack)
>begin
>   if(not empty(s)) then
>      -- pop the stack
>   else
>      raise underflow
>end pop;

>procedure pop(s: stack)
>begin
>   stack.top := stack.top - 1;
>   exception
>      when Constraint_Error => raise underflow
>end pop;

Note that in the second case the procedure is faster, since it doesn't
have to do the check first. Not only is it faster, it is safer, since
without using tasks you cannot guarantee that between the time you
checked and the time you popped it hadn't been popped elsewhere. For
both of these reasons, I'd say the second version is far better than
the first version, and that the original poster's thesis that exceptions
should be used rarely if ever has been contradicted by the very examples
provided to support his/her case!

P.S. You need an "end if" in the first example.
--
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