DIP 1006 - Preliminary Review Round 1

[Mathias Lang via Digitalmars-d]

On Wednesday, 12 April 2017 at 16:22:00 UTC, Lewis wrote:
>
> I have to ask the newbie question, just to make sure we're not 
> missing anything obvious. Why can't we fix invariants so that 
> they're pay-for-what-you-use? In other words, is there a way we 
> can make sure _d_invariant is never called (or early-outs) for 
> classes that don't use invariants?

There's no newbie question :)

Sadly it is not possible. Consider the following hierarchy:
```
class Mother : Object { public int i; public void myFunction () 
{} }
class Daughter1 : Mother { invariant () { assert(i != 0); } }
class Daughter2 : Mother {}
```

The `Mother` class needs to insert invariant checks at the 
beginning and the end of `myFunction` to account for the 
possibility of a derived class defining invariant (here 
`Daughter1`). However those checks are superfluous if no 
`invariant` is defined, as it's the case with `Daughter2`.
However the base class cannot know this in advance (because it 
might be in a library and already compiler, for example).