On Thu, 19 Jan 2012 22:31:08 -0500, Walter Bright
<newshou...@digitalmars.com> wrote:
On 1/19/2012 6:37 PM, Steven Schveighoffer wrote:
I have to disagree on some level with (1). It might not make a
difference in
determining there is a bug, but it makes a difference because failing
in the
out-condition gives you more information, even if the invariant is
broken. It
tells you which function broke the invariant.
No, it doesn't give you more information in the out condition.
Furthermore, the postcondition and the invariant check entirely
different state - the return value is not the same thing at all as the
instance state. There's no reason to believe that one is superior to the
other, nor that they are redundant.
If they are related, yes it does give you more information. The out
condition might not check the class data directly, but the error which
caused the invariant to fail could have also caused the out-condition to
fail. It's inconsequential to the *program* whether the out condition or
the invariant halts execution. But to the developer receiving the assert
message, it's much more helpful to see an out condition failing than an
invariant. If they are related, and caused by the same bug, it's that
much more helpful. Imagine you have 1000 lines of code that call 50 or so
different methods on a class. Any one of those calls in any one of those
methods could cause an invariant failure. But only one method call can
cause a specific out condition failure, and the lines of code that call
that function might be significantly less than 1000 (maybe a handful).
With almost no change to execution speed, semantics, or pretty much
anything, you will have saved the developer minutes to hours (under the
right circumstances, maybe even days or weeks) of crappy "how the hell do
I find this bug" time. I can't see a single drawback aside from the time
it takes to test the changes to the compiler.
even your second reason is flawed -- in order for there to be a noticeable
difference, one would have to make their invariant actually *change* the
object. Why do we support that? In fact, this change would help them
discover their flawed invariant code :)
-Steve
