VC++ Library Clashing Problem

Question

I am working on a C++ project that uses Qt (gui lib), VTK (graphics lib) and another library which is so obscure I won't mention its name and will instead call it LIB_X. The project uses Qt for the gui components and VTK (more precisely the QVTKWidget extension provided by VTK that supports Qt) for rendering geometry.. and it uses LIB_X to gather and manipulate geometry.

The problem is that it turns out that LIB_X actually uses VTK (where and how, I don't know, it's closed source). At first there was no problem, compiling with both libs linked was going fine, but at some point I called a certain (and highly needed) LIB_X function and compiling led to a bunch of 'blah blah something about a VTK lib/obj already defined in LIB_X dll' errors.

e.g. (and note this is with /FORCE:MULTIPLE so it's a warning here, let me know if you want the error without /FORCE:MULTIPLE and I'll post it):

1>LIB_X.lib(LIB_X.dll) : warning LNK4006: "public: __thiscall std::vector<double,class std::allocator<double> >::~vector<double,class std::allocator<double> >(void)" (??1?$vector@NV?$allocator@N@std@@@std@@QAE@XZ) already defined in vtkCommon.lib(vtkInformationDoubleVectorKey.obj);

I tried using /FORCE:MULTIPLE and it seemed to be a miracle at first, but I am getting random errors in code that would mostly give heap errors. I decided to remove all references to LIB_X from the main project and created a static lib that would handle all LIB_X stuff. I'm not a C++ expert, so I'm not certain how it handles lib clashing when you're using a pre-compiled lib, but I still received lib clashing errors when linking my static lib into my main project, so I still have to use /FORCE:MULTIPLE.

Once I had the static lib it seemed like the random errors had gone away, I was able to do a lot with LIB_X methods in the main project via the static lib, BUT out of nowhere, I added a new data member to my main project's class (a std::vector of doubles) and suddenly I was getting a heap error in one of my static library's methods. If I commented out the new data member, the static library's method would run fine. I hate to give the current error, because honestly I'm not sure if examining it will be worthwhile, but here it is anyway in case it can help:

note: it crashes to xutility on about line 151, pops up assertion: "file: dbgheap.c line: 1279 expression: _CrtIsValidHeapPointer(pUserData)"

The error comes after adding a vector vector double to a vector vector vector double, crashing on the push_back line:

std::vector<std::vector<double>> tmpVec;
for(srvl_iter = srvl.begin(); srvl_iter != srvl.end(); ++srvl_iter)
{
 tmpVec.push_back((*srvl_iter).getControlPoints());
}
this->_splines.push_back(tmpVec); //CRASH

It only started crashing here when I added a new data member to my main project (separate from the static lib!) Commenting out the new data member takes the error away.

std::vector<std::vector<std::vector<double>>> _geometry; 

So, /FORCE:MULTIPLE seems bad, I get random errors that just don't make sense to me. Are there other solutions? Am I screwed? Is there something I can do with LIB_X's linking of VTK?

Answer 1

I encountered a bunch of LNK4006 errors when linking my app to a library (call it library LIB_Y) that made heavy use of std::vector<std::string>, which I also did in my app. After a bit of experimenting I found one solution that worked -- wrap LIB_Y in a separate DLL that calls LIB_Y (LIB_Y_WRAPPER, say), and then link the main app against LIB_Y_WRAPPER.

To try out my suggestion you will need to:

1. Change your "static lib that handles all LIB_X stuff" from a static LIB project into a DLL project (which I will call LIB_X_WRAPPER).
2. Make sure the header files of LIB_X_WRAPPER don't include any of the LIB_X header files. This is really important because the wrapper needs to completely isolate your app from the data types declared in the LIB_X header files (such as std::vector<double>). Only refer to LIB_X's header files from within the source files of LIB_X_WRAPPER.
3. Change the declaration of all classes and functions in your static lib to ensure they are exported from the DLL (see this answer if you need details about exporting from a DLL).

This solution worked for me because it kept the instantiation (compiler generated functions) of the std::vector<std::string> class used by LIBY completely separate from the instantiation of std::vector<std::string> in my app.

As an aside, I suspect the cause of the crash you are seeing (you comment it is in the destructor of std::vector<double>) is because the instantiation of std::vector<double> in your app is different to that in LIB_X.

Answer 2

The commenting out is probably just random luck - if you are corrupting the heap you don't always see it right away but an stl vector will allocate and deallocate things left and right so it's no wonder it's finding the error.

Some libs require you include things in a certain order. I am not sure why exactly because to me it seems like giving up and saying you can't design a library properly, but it's the sad fact. So long as you don't include this lib_x anywhere that you include vtk it should be fine, though.

However, they might be fighting over some resource or using something improperly that makes it impossible for them to work together. If you are able to get the compile to work ok by segregating them and it still fails then yes you are just out of luck because it's just a failing in the way that this lib_x was designed and since it's so obscure it's not likely to have been thoroughly debugged against all uses. When something's not widely used it usually ends up being something that works on the developer's machine and project but not necessarily anyone else's.