Question

I converted a function to a template, and started getting this error. I must not be understanding a limitation of templates. Can someone tell me why this is broken?

I am receiving this error:

Undefined symbols:   'bool foo<int>(int const&, int const&)', referenced from:       _main in file1.o ld: symbol(s) not found 

When I link the following code. The code is simplified, but still fails. The first file contains:

#include <iostream> template <class T> bool foo (const T&, const T&);  int main () {   int left = 1;   int right = 2;    if (foo <int> (left, right))     std::cout << 'foo!' << std::endl;    return 0; } 

And the second file contains:

template <class T> bool foo (const T& left, const T& right) {   return true; } 

Answer 1

For the reason Uri gave, template methods are usually defined in the header file. Because yours is a function and not a method of a class, explicitly define it (in the header file which may be included by more than one CPP file) as static or inline.

Put this in your foo.h

template<class T> inline bool foo (const T& left, const T& right) {   return true; } 

Put this in your main.cpp

#include <iostream> #include 'foo.h'  int main () {   int left = 1;   int right = 2;    if (foo <int> (left, right))     std::cout << 'foo!' << std::endl;    return 0; } 

The cpp code now sees the whole declaration of the template function.

Other solutions are listed here: How can I avoid linker errors with my template functions?