Hi Marcus,
Your explanation seems logical to me to if I
understand it correctly. But I thought Microsoft's compiler are more
strict about C++ standards although they don't comply too much with it.
What it really means is that most of the times they will implement a
super set of ISO standard. They generally have some extension which
would not be part of the ISO standard , but apart from extensions rest
will be more ISO complaint than say g++.
Thanks,
Divick
Marcus Lindblom wrote:
> Divick Kishore wrote:
>
>> Hi Marcus,
>> thanks for the reply.
>>
>>
>>>> Yes, the warning is totally ignorable. We've been running with it
>>>> disabled for over a year now. :)
>>>>
>> Still that does not answer why VC7 gives those errors ? From a language
>> semantics perspective what really is wrong with that line?
>>
>
> VC7.1 isn't very ISO C++ conformant, although it a lot better than it's
> predecessors. I'm not 100% sure but I think that extern wasn't in the
> standard when they released 7.1, hence the compiler warns that you are
> using something that's not in the standard (Or the MSVC-dev team didn't
> have time to fix it, VS71 is not exactly a thoroughly tested and
> overworked product). The warning is quite nice, but since the compiler
> has another slew of isses that are not standard which it doesn't warn
> for, this one makes little sense.
>
> The 'extern' keyword was meant to be used to define that the template
> specialization is instantiated in another compilation unit (i.e.
> cpp-file), rather than the current one, to avoid code-bloat of each .cpp
> generating code for, say, std::vector<int>. (This is reduced to one set
> of machine-code by the linker, so it only affects the size of your
> .obj-files.)
>
> So, this is all very nice in theory, but due to some issues with the
> template instantiation mechanism (I can't remember the details, but I
> think Herb Sutter wrote an article about it, search on www.gotw.ca) it
> doesn't really work as intended anyway.
>
> I think Visual 8 gives the warning: "'extern' keyword not supported."
> instead, but I'm not sure. I do know that neither VC7 nor VC8 implements
> it properly (I recall reading it in the docs) and that you still get the
> correct code (OpenSG runs fine on both VC7 and VC8), it justs take
> longer to compile and link.
>
> When I want to do something like 'extern' and get results, I usually end
> up with putting the template-source in the cpp-file and doing explicit
> instantiation there, for all types that my executable will need. This
> means two things: 1. no inlining. 2. all template functions get
> instantiated, not just the ones that are used (this can be serious since
> some of your templated class's functions might only compile with certain
> types, etc.).
>
> Perhaps the gcc guys have managed to get it to work properly (or at
> least for OpenSG's application thereof) so that would also explain the
> difference in compile speed for OpenSG on *nix vs. windows/msvc, as well
> as why extern is being used.
>
> Cheers,
> /Marcus
>
>
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