[Issue 5023] New: Docs about order of execution of invariant and pre/post conditions

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=5023

           Summary: Docs about order of execution of invariant and
                    pre/post conditions
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Keywords: spec
          Severity: normal
          Priority: P2
         Component: websites
        AssignedTo: nobody at puremagic.com
        ReportedBy: bearophile_hugs at eml.cc


--- Comment #0 from bearophile_hugs at eml.cc 2010-10-09 06:35:21 PDT ---
This D2 docs page states:
http://www.digitalmars.com/d/2.0/class.html#invariants

> The invariant is checked when a class constructor completes, at the start of
> the class destructor, before a public or exported member is run, and after a
> public or exported function finishes. 


If preconditions, postconditions and invariants are pure functions then they
need to refuse the usage of functions like printf() inside them. If they aren't
pure (maybe because they must support printing and logging), then printing and
logging is a side effect, and their order of execution counts.

Even if preconditions, postconditions and invariants are pure functions, their
purpose is to stop the execution of the program when there's a bug, so during
debugging it's important to know the order of execuction of those contracts, to
understand where the bug is.

Either way, the order of execution of preconditions, postconditions and
invariants is important and it needs to be stated in the D documentation (and
it needs to be the same in different D implementations).

So I suggest to change the D docs like this:

> The invariant is checked when a class constructor completes, at the start of
> the class destructor, before a public or exported member is run (before its precondition),
> and after a public or exported function finishes (after its postcondition).

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