Undefined reference to function template when used with string (GCC)

Question

I need to write a templated function replace_all in C++ which will take a string, wstring, glibmm::ustring etc. and replace all occurrences of search in subject with replace.

replace_all.cc

template < class T >
T replace_all(
        T const &search,
        T const &replace,
        T const &subject
) {
        T result;

        typename T::size_type done = 0;
        typename T::size_type pos;
        while ((pos = subject.find(search, done)) != T::npos) {
                result.append (subject, done, pos - done);
                result.append (replace);
                done = pos + search.size ();
        }
        result.append(subject, done, subject.max_size());
        return result;
}

test.cc

#include <iostream>

template < class T >
T replace_all(
        T const &search,
        T const &replace,
        T const &subject
);

// #include "replace_all.cc"

using namespace std;

int main()
{
        string const a = "foo bar fee boor foo barfoo b";
        cout << replace_all<string>("foo", "damn", a) << endl;
        return 0;
}

When I try to compile this using gcc 4.1.2

g++ -W -Wall -c replace_all.cc  
g++ -W -Wall -c test.cc  
g++ test.o replace_all.o  

I get:

test.o: In function `main':
test.cc:(.text+0x13b): undefined reference to `
   std::basic_string<char, std::char_traits<char>, std::allocator<char> >
   replace_all< std::basic_string<char, std::char_traits<char>, std::allocator<char> > >(
       std::basic_string<char, std::char_traits<char>, std::allocator<char> > const&,
       std::basic_string<char, std::char_traits<char>, std::allocator<char> > const&,
       std::basic_string<char, std::char_traits<char>, std::allocator<char> > const&
   )
'
collect2: ld returned 1 exit status

But when I uncomment #include "replace_all.cc" in test.cc and compile this way:

g++ -W -Wall test.cc

The program links and produces expected output:

damn bar fee boor damn bardamn b

Why linking fails and what can I do to make it work?

Answer 1

You can't link templates as compiler don't know which code to generate before someone tries to use ( instantiate ) templates.

You can "ask" compiler to instantiate template if you knows which types are you going to use or if you know that they are limited.
If you want - put this to your .cc file:

template std::string replace_all( std::string const& search,
                                  std::string const& replace,
                                  std::string const& subject );


template glibmm::ustring replace_all( glibmm::ustring const& search,
                                      glibmm::ustring const& replace,
                                      glibmm::ustring const& subject );