Linker Problem with Msvc

[Thiago Macieira]

Christian Ehrlicher wrote:
>Hi,
>
>once again an ugly linker problem with Msvc and kio (class MetaData in
>global.h). It's the same like described here:
>http://lists.trolltech.com/qt-interest/2005-12/thread00622-0.html

And again it's caused by the fact that the MS compiler & linker don't 
properly understand the concept of common functions and weak symbols. The 
C++ standard allows for this, so both the linker and dynamic linker in 
Win32 platforms should be modified to allow for them.

The problem here is akin to inline functions and I wouldn't be surprised 
if it showed up with them. A function is defined in one .h and exported 
from it (__declspec(dllexport)). It is inlined in two different objects 
or shared objects. Therefore, both export the function.

When linking (statically or dynamically), the linker is expected to merge 
those two functions. This isn't happening.

As a side note, gcc has a switch called -fvisibility-inlines-hidden for 
similar issues.

>According to the reporter, I have to define something similar to this:
>#ifdef Q_OS_WIN
>template class __declspec(dllimport) QMap<QString, QString>;
>#endif

Which is ugly.

Can't you add a simple, forward declaration using Q_CORE_EXPORT? I.e., 
lose the #ifdef and replace the __declspec with Q_CORE_EXPORT.

In fact, come to think of it, one could propose to make ALL forward 
declarations include the proper EXPORT attribute. I am not sure if this 
would be wise, since classes can change libraries and this would require 
updating the header files (all of them) when it happened.

But we could do it for all the templates, though.

>But before I add this ugly hack, I wanted to know if MetaData has to be
>exported or not. If not, I could skip this.
>Or has someone another idea to fix this?

It's possible to do without a new class called MetaData. We can simply 
typedef QMap<QString, QString> to it and move the operator+= to a 
non-member function.

BTW, shouldn't we be using a QHash instead?

-- 
  Thiago Macieira  -  thiago (AT) macieira.info - thiago (AT) kde.org
    PGP/GPG: 0x6EF45358; fingerprint:
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4. And æfter se scieppend ingelogode, he wrát "cenn", ac eala! se 
rihtendgesamnung andswarode "cenn: ne wát hú cennan 'eall'. Ástynt."
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