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Subject: Re: [boost] [system] Header-only Boost.System by default ?
From: Damien Buhl (damien.buhl_at_[hidden])
Date: 2017-10-10 19:54:04

On 10/10/2017 09:32, Andrey Semashev via Boost wrote:
> [...]
>> Is there really no guarantee that category always get the same address
>> across TUs ?
> It will have the same address as long as get_basic_error_category() is
> called within the same module (dll, so, exe, etc.) If this function is
> compiled in different modules then there will be multiple instances of
> the category with different addresses.
> On Linux and probably other Unix-like systems this can be solved by
> exporting the category instance, which is the default. I don't think
> this can be done on Windows - you'd have to export
> get_basic_error_category for that instead and it will make this code
> less friendly to header-only libraries.

Thanks for these explanations.
Is there any unit-tests or are any open program relying on the
uniqueness of error_category across modules ?

Am 10.10.2017 um 10:21 schrieb Groke, Paul via Boost:
>> [...]
> And even if the "same address" thing was solvable (which I think it
> is, although not pretty and it would require interface changes), there
> is the problem with unloading DLLs/SOs. You'd have to pin every module
> that contains error categories. Which IMO is a side effect that you
> really don't expect and want, especially from a header-only library.
I must be missing something about internal linkage. This problem should
already exists with the current implementation no ? If you have an
error_category singleton instance in a TU, then unloading a dll
containing this TU should also lead to having the error_category refer
to some incorrect memory location then.

Do you know of any example code doing this ?

Damien Buhl
Software Developer
+33 6 77 43 10 05

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