Temporary suspension of disbelief (invariant)

[Stanislav Blinov]

  27.10.2010 4:00, bearophile wrote:
> I have not asked this in the D.learn newsgroup because I think it may be a bit interesting for other general people too.
>
> In D contract programming the invariant is not called before/after private methods. I think that in some cases you may want to disable the invariant even in some public methods. If your class/struct has a public method that invalidates the state of the class instance, and one public method that fixes the class instance state (I am thinking about certain data structures where in a first phase you may add many items, and then you ask the data structure to clean up itself and become coherent. This may avoid some computations), I have found this way to implement it:
If I get this right, then it is by design that your class may have 
several general logical states: e.g. "initializing" and "coherent". 
Given this, I don't see why you'd want to disable invariant checks 
rather than modify those checks instead to validate current logical 
state. In fact, that "ghost" field simply serves as a flag for invariant 
showing which logical state it should enforce. The fact that states are 
'logical', e.g. are different while represented by the same physical 
fields doesn't always rule them out as, uh, class states: you could as 
well have two separate inner classes that perform initialization and 
polishing, each with its own invariant. Then you could use those inner 
classes' private methods (without causing their invariant checks), but 
in main class' invariant perform an assert on them to ensure their state 
is valid.

IMHO, proper invariants should be strict and act by the numbers at all 
times, otherwise there's little to no gain in using them at all.