Upon running make, I get the following output:

g++ -w -std=c++14 -o chainDriver chain.o chainDriver.o 
Undefined symbols for architecture x86_64:
  "void chain::append<int>(int)", referenced from:
      _main in chainDriver.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [chainDriver] Error 1

I am running this on a Macbook Pro, mid-2010. I'm not sure if it's a hardware-specific issue, but I'd like any fix to this to be portable. Strangely enough, this compiler error didn't occur when everything was just crammed into one single .cpp file. Now that I've separated out my code into a .h and driver file, though, I'm getting this bad boy.