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 );