Re: [boost] [contract] move operations and class invariants

Subject: Re: [boost] [contract] move operations and class invariants
From: Peter Dimov (lists_at_[hidden])
Date: 2017-12-01 04:20:52

Lorenzo Caminiti wrote:

> > read() can fail, so you can drop its "valid" precondition and just fail 
> > when the handle is invalid.
>
> Calling read() on a moved-from object so violating its is_valid() 
> precondition is a bug, not a run-time error while reading the file. The 
> precondition legitimately remains in place to catch bugs, even after it is 
> noted that read() can fail because of file-system or other run-time 
> errors.

On one hand, you want to have a precondition.

On the other, you say that if you have the precondition, the class would be 
crippled and not very useful:

>> 2. How useful is a class like the one above with "crippled" invariants 
>> and is_valid() preconditions on all its useful public methods like 
>> read()? The answer seems to be: not very useful.

These are contradictory. Either the class that has preconditions is crippled 
and not very useful, in which case we drop the preconditions as I suggest; 
or it's useful, in which case we keep them.