C++ Shared Library with Templates: Undefined symbols error

Question

I\'m trying to link to a shared library with a template class, but it is giving me \"undefined symbols\" errors. I\'ve condensed the problem to about 20 lines of code.

Answer 1

The compiler has to see all the code for a template, so it can generate the appropriate code for the actual type you want to use. So you should place all the code in your .h. file.

Answer 2

You need to include the implementation of the template classes in the header files as well. This is a constraint of templates in C++. So either include shared.cpp from main (#include ) or just move the code from shared.cpp in shared.h

Answer 3

In addition to the other answers, you can explicitly instantiate template classes. This is only useful if you know beforehand what types the template parameters may assume. You instantiate the template with all these types in the library.

For your example to compile, just add the following to the end of shared.cpp:

// Instantiate myclass for the supported template type parameters
template class myclass<int>;
template class myclass<long>;

This instantiates the template with Type=int and places the instantiated code in the shared library. Add as many explicit instantiations as you need, for all the types you need.

Again, if you want to be able to instantiate the template with any arbitrary Type parameter, then you must add the definitions to the header file, so that the compiler knows the source code of the template when instantiating it in other compilation units.

Answer 4

Template function definitions must reside in header files. Move the definitions from shared.cpp to shared.h.

So, you can't compile this to a shared library and then link to it. It just doesn't work like that.