Question

I was starting to code a library, and decided to do a test, but I’m getting the error in the question title (Mac OSX, gcc-4.7.1):

tlib.cpp:

template <typename T>
T dobra(const T& valor){
  return valor*2;
}

tlib.h:

template <typename T>
T dobra(const T& valor);

test2.cpp:

#include "tlib.h"
#include <iostream>

using namespace std;

int main (int argc, char const *argv[])
{ 
  double b = dobra<double>(10);
  cout << b << endl;
  return 0;
}

Compiling:

no25-89:CPROP canesin$ g++ -dynamiclib -Wall -std=c++11 tlib.cpp -o libdobra.so
no25-89:CPROP canesin$ g++ test2.cpp -Wall -std=c++11 -o test2 -L. -ldobra
Undefined symbols for architecture x86_64:
  "double dobra<double>(double const&)", referenced from:
      _main in cctLJGqf.o
ld: symbol(s) not found for architecture x86_64
collect2: error: ld returned 1 exit status
no25-89:CPROP canesin$ 

Answer 1

A fact of life in C++ is that you must include a complete implementation of a template in every compilation unit it’s used in, or restrict yourself to specific instantitions.

In practice, this means that you either:

1. Put what you’ve got in tlib.cpp into tlib.h. This is the most common solution.

2. Restrict yourself to only ever using (say) dobra<double>, and put an explicit instantiation into tlib.cpp:

   template double dobra<double>(const double& valor);