Re: [boost] expected/result/etc

Subject: Re: [boost] expected/result/etc
From: Emil Dotchevski (emildotchevski_at_[hidden])
Date: 2016-01-28 19:57:16

On Thu, Jan 28, 2016 at 3:44 PM, Domagoj Saric <dsaritz_at_[hidden]> wrote:

> On 28.1.2016. 21:02, Emil Dotchevski wrote:
>
>> Have you considered using Boost Exception? It lets you add arbitrary data
>> to exception objects, even after they have been thrown, regardless of
>> their
>> type -- see
>> http://www.boost.org/doc/libs/release/libs/exception/doc/error_info.html.
>> The general idea is to adopt a strategy where the exception objects are
>> augmented with any, even platform-specific relevant data, which then the
>> catch site can analyze to choose the correct handling.
>>
>
> Hopefully these efforts will bring us to a solution that will do away with
> the, IMO, false dichotomy between "errors" and "exceptions" (<system_error>
> vs <exception>) - rather we will start thinking only about "errors"/error
> objects which can be transfered in two different ways - like a ball in a
> rugby game - it can be passed 'on foot' hand to hand or it can be thrown
> over intermediate players to its final destination (only with errors its in
> reverse - they are mostly passed/returned or thrown 'backwards':).
> IOW things like error_info will be applicable to 'the' standardized error
> type(s) that functions also return, not just throw.
>

You could theoretically pass exceptions function-to-function by
exception_ptr but that seems backwards, no pun intended. :)

Anyway, my point is that there are two ways one can go about solving the
problem of transporting error codes. One is what seems to be the path
you're taking, where you design a common API that can fit all error codes
on all platforms. On the other hand, the Boost Exception approach sidesteps
this difficult problem.

Let's take socket errors for example, which on POSIX are communicated by
the OS through errno, while on Windows there is the GetLastError function.
Using the Boost Exception approach, you would always store the errno into
any socket exception object, and on Windows you'd also store the
GetLastError code. At the catch site, regardless of the platform, you can
put in logic that takes correct action based on the presence or absence of
any particular error_info:

catch( socket_error & e )
{
    int const * e1=get_error_info<errinfo_errno>(e);
    assert(e1!=0); //errno must be present in all socket_error objects
    if( int const * e2=get_error_info<errinfo_GetLastError>(e) )
    {
        //Both errno (*e1) and GetLastError (*e2) codes available.
    }
    else
    {
        //No GetLastError, use errno only.
    }
}

Emil