Undefined reference from templated function (C++)

Question

main.o: In function `main':
main.cpp:(.text+0x2f): undefined reference to `Foo<int>::display(int)'
collect2: ld returned 1 exit status

caused by

g++ -c *.cpp && g++ *.o -o foo

with foo.hpp:

#ifndef FOO_H_
#define FOO_H_

template<typename T>
class Foo {
  private:
    T ft_;
  public:
    Foo(const T & ft) : ft_(ft) { }
    Foo display(T x);
};

#endif

foo.cpp:

#include "foo.hpp"
#include<iostream>
using namespace std;

template<typename T>
Foo<T> Foo<T>::display(T x) {
  // do some stuff - not too relevant
  cout << "[" << x << "]";
  Foo<T> res(x);
  return res;
}

and main.cpp:

#include<iostream>
#include "foo.hpp"
using namespace std;

int main() {
  Foo<int> f(42);
  Foo<int> g = f.display(39);
  return 0;
}

Why?!

P. S. Works with inlined function definitions. Troubles come whenever declaration and definition of a function is split into two files...

You forgot to read the page in your C++ book that told you not to define function templates in a `.cpp` file. — Lightness Races in Orbit, Feb 14 '13 at 14:08
http://stackoverflow.com/questions/495021/why-can-templates-only-be-implemented-in-the-header-file — Lightness Races in Orbit, Feb 14 '13 at 14:10
See also http://www.parashift.com/c++-faq/templates-defn-vs-decl.html — Jonathan Wakely, Feb 14 '13 at 14:14

score 3 · Accepted Answer · edited Feb 14 '13 at 14:18

3

In C++ you need to put the definition of templated methods and functions into the header file.

edited Feb 14 '13 at 14:18

Lightness Races in Orbit

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answered Feb 14 '13 at 14:08

Ivaylo Strandjev

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1

And what about in C++11? – Lightness Races in Orbit Feb 14 '13 at 14:10
@LightnessRacesinOrbit hmm it seems I have been confused about extern templates meaning... I think it was proposed that one can define templates in the `.cpp` file and thought it got in. Removing that from the answer. – Ivaylo Strandjev Feb 14 '13 at 14:17
Perhaps you're thinking about `export`, which was in fact _removed_ from C++11, but never worked anyway. – Lightness Races in Orbit Feb 14 '13 at 14:18
ThanX! It confused me since I've seen it used this way in one of my C++ book... – mathemage Feb 14 '13 at 14:21

Undefined reference from templated function (C++)

1 Answers1