Re: [boost] expected/result/etc

Subject: Re: [boost] expected/result/etc
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2016-02-10 20:02:26

On 11/02/2016 12:23, Emil Dotchevski wrote:
>> That's still a fuzzy line though.  In the delete file case, the operation
>> of deleting the file cannot proceed because the file is already absent.
>> However the post-condition of "the file no longer exists" is still met.  Is
>> that success or failure?
>
> "Proceed", I mean the caller. Let's say you have code which opens the file
> then reads from it. The caller can not proceed to reading if the file
> couldn't be opened. So, the postcondition of the open operation is that the
> file was successfully opened. Similarly, the postcondition of delete_file
> is that the file does not exist, because presumably the caller of
> delete_file can't proceed if the file still exists.

As I said before though, there could be some applications (eg. certain 
types of databases) which can't proceed if that particular deletion 
action was not the cause of the file ceasing to exist (ie. if some other 
process deleted it in advance).  It's not as clear-cut as you seem to be 
suggesting.

> If you know that the queue is never supposed to be full, that is, if the
> full queue indicates a bug in your code, then you should assert rather than
> throw. Throwing is when you expect the program to successfully recover from
> an anticipated (by the programmer) failure.

The library can't know that, though (unless it was designed to be 
unbounded, so can't be full unless memory allocation fails), so it must 
pass this state out to the application to deal with, and an exception is 
*not necessarily* the right way to do so.  (And neither is 
not-an-exception, for that matter.)

If you *had* to pick one or the other, though, in cases where failure is 
unusual (such as failing to push to an unbounded queue) an exception 
makes more sense, and in cases where failure is not unusual (such as 
failing to push to a bounded queue) a return status makes more sense.

I think it's been well established that exceptions are great for dealing 
with unexpected failure (since you have to actively ignore them), but 
not so great for expected failure (since they can do heavy things like 
capturing call stacks and stack unwinding).

> It depends. For example, if this is a keyboard buffer queue, then probably
> you'd return a status to indicate that there's no more space, and the
> caller will "beep" to tell the user to slow down with the typing. In this
> case it would be annoying for the caller to have to catch an exception. On
> the other hand, if it's a job queue, where the caller expects all submitted
> jobs to be queued and at some point completed, then an exception is more
> appropriate, so the immediate caller wouldn't have to care (push won't
> return), while some context higher up the call stack can perhaps retry the
> whole sequence of jobs, or notify the user that the operation failed.
>
> We can't discuss this stuff in the abstract. Defining correct
> postconditions depends very much on the specific API being designed.

Except that for things like generic data structures, the implementing 
library can't possibly know what the application is going to put in them 
or do with them.

So you have to deal in the abstract, which is why you often need both 
throwing and non-throwing API in those sorts of cases -- only at the 
application level can you finally decide which one to call.