Subject: Re: [boost] version conflicts: is there a solution?
From: Ilya Bobir (ilya.bobir_at_[hidden])
Date: 2009-06-27 15:50:21
Ilya Bobir wrote:
> Frank Mori Hess wrote:
>> On Thursday 25 June 2009, Edward Diener wrote:
>>>> Can you point to anything in the standard that supports your position?
>>> Where in the standard do you see any mention of an ODR violation
>>> occuring where there are no header files being distributed for inclusion
>>> by a source file which would create such a violation ?
>>
>> There is the 3rd paragraph of the "one definition rule" section:
>>
>>
>> "Every program shall contain exactly one definition of every
>> non-inline function or object that is used in that program; no
>> diagnostic required. The definition can appear explicitly in the
>> program, it can be found in the standard or a user-defined library, or
>> (when appropriate) it is implicitly defined (see 12.1, 12.4 and 12.8).
>> An inline function shall be defined in every translation unit in which
>> it is used."
>>
>>
>> Note they include "user-defined library".
>>
>
> I think that in order to see the whole picture you need to explicitly
> define what a "definition" is. ISO 14882-2003 3.1 paragraphs 1st and 2nd:
>
> > A declaration (clause 7) introduces names into a translation unit or
> > redeclares names introduced by previous declarations. A declaration
> > specifies the interpretation and attributes of these names.
> >
> > A declaration is a /definition/ unless it declares a function without
> > specifying the function’s body (8.4), it contains the extern specifier
> > (7.1.1) or a linkage-specification24) (7.5) and neither an initializer
> > nor a function-body, it declares a static data member in a class
> > declaration (9.4), it is a class name declaration (9.1), or it is a
> > typedef declaration (7.1.3), a using-declaration (7.3.3), or a
> > using-directive (7.3.4).
>
> Unless your "user-defined library" include headers you can not introduce
> names into a translation unit. And even if it does include headers the
> ODR applies only to the stuff you have in that headers and not to the
> "rest" of the library. It means that unless you have the Boost headers
> somewhere in the header part of your library there is no way to violate
> the ODR as it applies to the translation units only.
>
> The linkage of compiled translation units is described in a separate
> section and has its own rules. Described in section "3.5 Program and
> linkage". While there is something similar to ODR there, see paragraph
> 9, but I do not think that it will apply to your case, unless you
> explicitly declare parts of boost as having external linkage thus making
> them visible to the linker during the linking process.
>
Artyom wrote:
> There are many problems and this is not as simple as you think.
> Because if you have two versions of foo() defined in header
> files they may be exported and... collide.
Oh, according to ISO 14882-2003 3.5 paragraph 4 all functions, names
classes and some other type of names defined in a namespace scope have
external linkage by default. It means that you will violate this rule.