Re: VC++2003: Methods implemented outside class body are never inlined for templates that are instantiated with __declspec(dllimport)

From: Larry Brasfield (donotspam_larry_brasfield_at_hotmail.com)
Date: 03/15/05 

Date: Tue, 15 Mar 2005 15:06:16 -0800

"Felix I. Wyss" <felixw@inin.com> wrote in message
 news:OgBryeaKFHA.572@tk2msftngp13.phx.gbl...
> Good Afternoon,
Greetings.
> I recently noticed that some very simple methods of a template declared and
> used in a DLL library get inlined when used by the DLL itself, but not by
> other DLLs and EXEs. After some investigating, I narrowed this down to a
> very odd behavior (bug?) of the VC++.NET 2003 compiler: If a class that is
> declared as __declspec(dllimport) derives from a template, that template's
> methods are never inlined, even if declared with the "inline" specifier.
> Methods that are defined within the class body are correctly inlined by the
> compiler. The same is true to templates explicitly instantiated for a type
> as "template class __declspec(dllimport) Template<Type>;".
>
> As this is a bit tricky to explain, consider the following code:

[Cut 260 lines of C++, text, and assembler output.]

Whew! I think it might be simplest to just take your
word for what is happening since you appear to have
done your homework, (except for reducing the code
to a minimal example!)

First, the 'inline' keyword is strictly an advisory to the
compiler. There is no requirement in the C++ standard
that the compiler inline whatever is so marked, nor is
the compiler precluded from inlining code not so marked.

So 'bug?' cannot be the issue.

One might make an argument (albeit a complex one)
that this is a quality of implementation issue. I will
take the side of the compiler implementers on this one.

Why would you use a DLL if it was not supposed to
be interchangable with newer versions or (in the case
of plugin architectures) a different implementation?
Inlined code from class methods is first and foremost
an implementation detail that is exposed only for the
sake of execution efficiency. DLL's are primarily
motivated by a desire to separate interface from
implementation, and bind the two at nearly the latest
possible moment. (See "delay loading".) So, if the
compiler feature designer(s) elected to not add some
syntax for saying "Bind my implementation into the
DLL client code." while still allowing inlining for the
benefit of the implementation of the DLL itself, I can
see that as an arguably sensible decision.

If you cannot get happy with it, I would suggest that
a few friends with whatever privileged access those
would-be inlined methods enjoyed could be made
to accomplish the same effect. That approach will
degrade encapsulation somewhat, but that clearly
is something you are willing to tolerate. 

-- 
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

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