Re: [boost] [safe_numerics] One more review

Subject: Re: [boost] [safe_numerics] One more review
From: Antony Polukhin (antoshkka_at_[hidden])
Date: 2017-03-11 17:41:37

2017-03-11 20:22 GMT+03:00 Robert Ramey via Boost <boost_at_[hidden]>:
> On 3/11/17 7:52 AM, Steven Watanabe via Boost wrote:
>>
>> AMDG
>>
>> On 03/11/2017 02:00 AM, Antony Polukhin via Boost wrote:
>>>
>>>
>>> I'd like to see an additional statefull ExceptionPolicy that remembers
>>> that an UB was triggered, but does not throw at all. Here's how it
>>> could be used:
>>>
>>
>>   This also requires an extra function to combine two
>> ExceptionPolicies for binary operators.  You'll
>> run into problems with comparison operators,
>> though, as a bool can't hold an ExceptionPolicy.
>
>
> Couldn't the addressed by a variation of the "ignore exception" policy which
> is meant to just return?
>
> A simple extension to log such errors could be crafted from this idea and
> would make a great example on how to make one's own exception policy.
>
> What is missing from this idea?

State of the Policy is missing. In many cases it could be useful to
have a Policy with state to be able to get the UB info. This may be
useful for different try_* functions that must not throw, usually are
in hot path and rarely trigger UB:

bool try_do(int x_native) noexcept {
    bool was_an_error = false;
    safe<int, native, lazy> x(x_native, &was_an_error);
    // remembers that error was triggered in the `was_an_error` variable
    ++x;
    // ... other operations on `x`
    // ...

    // `x` is bad, after comparison `y` is also bad
    safe<int, native, lazy> y = x; // also copies pointer to `was_an_error`
    if (x < y) {
        // ... other operations on `y`
    }

    return was_an_error;
}

Throwing and catching an exception in such function may affect
performance. Rewriting the whole function using functions from
include/checked.hpp may take a lot of effort and will make the
function harder to read.

But this is more like a feature request, not a blocker to the library
acceptance. looks like they could be added later.

-- 
Best regards,
Antony Polukhin