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  Using std::map Across DLL Boundaries
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Note: This problem only occurs with the Microsoft STL implementation!

I don't know how many of you have run into this problem, but it certainly annoyed the heck out of me. The scenario was this:

I was writing a plugin/extension system and used a STL map for my plugin registry, basically mapping classnames to factories which can create that class, i.e. pretty standard stuff. One or several plugin classes could be stored in a DLL and my system traverses a user-defined directory and adds all the plugins contained in DLLs to its registry. Every DLL had a single entry function which would register the DLLs' content with the plugin system.

Now, I thought I could just pass the STL map to the DLL and be done with it, but that seemed to be a pretty bad idea ... it didn't work ... the program crashed when it tried to touch the map which I passed to the DLL. Or rather, it crashed when I tried to insert something into the map. Now, at first I thought I was doing something dumb, but when I switched to STLPort (another STL implementation) the problem vanished. In other words, there had to be something wrong with the Microsoft STL implementation. It turns out, that the code for the tree implementation (which is used to represent the map) uses references to static variables ... this is not so nice, since the actual values of these differ from executable to DLL. Therefore you couldn't just use an executable-side map in a DLL. Now, one solution to this is to switch to STLPort which you can find here:

http://www.stlport.org/

This library is really nice, but not everyone might have the chance to migrate to another STL implementation (for God knows what reasons).

A search on the web turned out the following site:

http://www.dinkumware.com/vc_fixes.html

There you can find some bugfixes for the VC++ header files. The file 'xtree' is the one you need to replace (always make backups!!!) in your VC++ include directory in order to be able to use std::map across DLL boundaries. It also fixes some multithreading issues with the xtree-derived STL classes.

Hope this tip helps somebody,

Marco Koegler (MK42)

The zip file viewer built into the Developer Toolbox made use of the zlib library, as well as the zlibdll source additions.

 

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