Undefined references to member functions of a class template

Question

I want to use iterators in template class method. Here is my code: (testclass.h)

template<typename T, typename container>
class TestClassX
{
public:
    void gen(typename container::iterator first );
};

and file testclass.cpp:

template<typename T, typename container>
void TestClassX<T, container>::gen(typename container::iterator first)
{

}

When i try to run it:

TestClassX<unsigned, std::vector<unsigned> > testx;
testx.gen(it);

I get an error:

Error:undefined reference to `TestClassX<unsigned int, std::vector<unsigned int, std::allocator<unsigned int> > >::gen(__gnu_cxx::__normal_iterator<unsigned int*, std::vector<unsigned int, std::allocator<unsigned int> > >)'

I use mingw32 4.4

I want to have a class that can write to different containers like std::vector, std::list, QVector or QList all that have STL-style iterators.

Answer 1

Template class methods NEED NOT be defined in the header files. But if you do this you need to define a separate compilation unit (for example templates.cpp) and in that you include the source code file of the template class (eg. #include "container.cpp" // the .cpp NOT the .hpp file) then you need to define the instances of the templates that you are using (eg. template class Container;). You also need to define the object for the template class (eg Link). In this particular case, since we are using a pointer to this object (eg Link*, in Containter ) we merely need to 'forward declare' that object.

Here is the full template.cpp file. Which you would compile and link in with the rest of the code.

class Link;
#include "Container.cpp"    // use the source code, not the header
template class Container<Link*>; 

I like using this method because it prevents the compiler from generating template class instances automagically and lets you know when it can't find it.

Compile with gcc using the option -fno-implicit-templates.

When you build everything will be compiled as normal but then the collector will recompile the templates.cpp file for all the objects that use the template.