Re: precondition and post condition check

I have taken time to look through the referred materials, they are about
AOP, contract based engineering... but not specific to pre- and post-
condition pattern.

Actually, why I ask this question is from the reading of the article. I want
to know what pre- and post- condition pattern is mentioned below, but this
concept is not covered with sample in this article. I did some exercises to
make my hands dirty and post my code here. And I did some search, but failed
to find related resource about this topic.

Any comments or recommended specific links about pre- and post- conditions?

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First, note that the base class is now in complete control of its interface
and policy, and can enforce interface preconditions and postconditions,
insert instrumentation, and do any similar work all in a single convenient
reusable place - the nonvirtual interface function. This promotes good class
design because it lets the base class enforce the substitutability compliance
of derived classes in accord with the Liskov Substitution Principle[3], to
whatever extent enforcement makes sense. If efficiency is an issue, the base
class can elect to check certain kinds of pre- and postconditions only in a
debug mode, for example via a non-debug "release" build that completely
removes the checking code from the executable image, or via a configurable
debug mode that suppresses selected checking code at runtime.
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