Re: [boost] expected/result/etc

Subject: Re: [boost] expected/result/etc
From: Domagoj Saric (dsaritz_at_[hidden])
Date: 2016-02-07 13:01:17

On 29.1.2016. 14:27, Niall Douglas wrote:
> On 28 Jan 2016 at 10:32, Michael Marcin wrote:
>
>> If you are going to handle the error in non-generic code very close to
>> the caller, I contend there is no need to actually call make_error_code
>> and wrap that enum into a std::error_code. The enum is typically 4
>> bytes, a std::error_code is usually 16.
>>
>> Certainly many times it doesn't matter at all but there are also times
>> where it would prevent me from having a good faith argument that this is
>> a strict improvement over a more C-style error handling approach.
>
> Unlike exception_ptr, error_code has no global memory effects and is
> elidable by the optimiser. Assuming there are no calls to extern
> functions between the return of an error_code and the handling of it,
> you will find your nominal 16 bytes reduced to a direct test of the
> state of errno/GetLastError which is of course no overhead at all.

And only if the function returning the error_code is inlined (or if the 
function has internal linkage and your compiler is 'very smart' and sees 
that the error_code is unused in all call sites and uses a custom 
calling convention that eliminates the error_code)...a good result<T, E> 
solution would produce optimal code even on ABI boundaries (e.g. a 
function returning a result<iterator_range, errno_t> exported from an 
x64 .so would pass the result in registers)...

-- 
"What Huxley teaches is that in the age of advanced technology, 
spiritual devastation is more likely to come from an enemy with a 
smiling face than from one whose countenance exudes suspicion and hate."
Neil Postman