Getting “undefined symbols for architecture” error when separating some functions from main.cpp, what to do?

Question

The story

My main.cpp is getting really cluttered so I decided to separate other functions from it (some helper functions for IO operations in main() ).

The problem

The files compile fine when the functions are within main.cpp . When I put them to a .h - .cpp pair, I get the following compiler error.

The line I use:

g++ -I ./headers/ ./definitions/*.cpp -o main.o main.cpp

The error:

    Undefined symbols for architecture x86_64:
      "void showSettings<double>(std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>,
std::__1::allocator<char> >,
std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>,
std::__1::allocator<char> > > > const&,
std::__1::vector<std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, 
std::__1::allocator<std::__1::basic_string<char, std::__1::char_traits<char>, 
std::__1::allocator<char> > > > const&, int, int, double, double, int, int, int, int, int)", 
referenced from:
          _main in main-7k1oIa.o
    ld: symbol(s) not found for architecture x86_64

The only working solution right now is to bring the functions back to main.cpp . I've included all necessary headers, using namespace std and prepending std:: to vector and string but the error keeps showing.

If it's of any help, the code that appears in the error is:

template <typename inputType>
void showSettings(  const vector<string> &a,
                    const vector<string> &b,
                    int eq,
                    int meth,
                    inputType root1,
                    inputType root2,
                    int sigs,
                    int showPerLoop,
                    int plotRoots,
                    int loopMode,
                    int minLoops)

Answer 1

Templates are only instantiated when called, so if no function in your secondary .cpp file calls showSettings with a <double> , then the function wasn't created.

Generally, you want to leave template functions in a .h file so that the actual definition is visible when needed.

(See also: Why should the implementation and the declaration of a template class be in the same header file? )