Subject: Re: [boost] [system] Would it be possible to trial a breaking change to Boost.System and see what happens?
From: Peter Dimov (lists_at_[hidden])
Date: 2018-01-14 16:30:11
Niall Douglas wrote:
> If you link all your DLLs with the static MSVC runtime so each gets a copy
> of a system_category implementation, _Immortalize still ensures that only
> one instance ever appears to exist in the process. So each error category
> always gets its unique address in the process. At least, this is what I've
> been told.
This was an interesting claim so I looked into it.
No, categories from DLLs using the static runtime do not have the same
address, and _Immortalize doesn't help with that.
C:\Projects\test_system_error>bin\msvc-14.1\debug\runtime-link-static\single_instance_test.exe
e1: 'system:0'; { 0, 560F6B00 }
e2: 'system:0'; { 0, 57E86B00 }
No errors detected.
They do, however, compare equal.
The equality comparison simply makes sure that all system categories compare
equal:
bool operator==(const error_category& _Right) const _NOEXCEPT
{ // compare categories for equality
return (_Addr == _Right._Addr);
}
where
/* constexpr */ error_category() _NOEXCEPT // TRANSITION
{ // default constructor
_Addr = reinterpret_cast<uintptr_t>(this);
}
but
protected:
uintptr_t _Addr;
enum : uintptr_t
{ // imaginary addresses for Standard error_category objects
_Future_addr = 1,
_Generic_addr = 3,
_Iostream_addr = 5,
_System_addr = 7
};
Perhaps we need to do something similar in Boost.System; the combination of
link=static and runtime-link=shared is disallowed at Boost Jamroot level,
but it could still help the "header-only in DLLs" case.