Storing C++ template function definitions in a .CPP file?

Question

I have some template code that I would prefer to have stored in a CPP file instead of inline in the header. I know this can be done as long as you know which template types will be used. For example: __.h file__ class foo { public: template <typename T> void do(const T& t); }; __.cpp file__ template <typename T> void foo::do(const T& t) { // Do something with t } template void foo::do<int>(const int&); template void foo::do<std::string>(const std::string&); Note the last two lines - the foo::do template function is only used with ints and std::strings, so those definitions mean the app will link. My question is - is this a nasty hack or will this work with other compilers/linkers? I am only using this code with VS2008 at the moment but will be wanting to port to other environments.

Answer 1

You are basically trying to do a template specialization on ints and std::strings. The way you normally write this is as follows:

#ifndef __FOO_H__

#define __FOO_H__

#include <iostream>

class foo {

public:

foo() = default;

template <typename T>

void doit(const T& t);

};

#endif

template <typename T>

void foo::doit(const T& t) {

std::cout << "foo::do<T> has a " << t << "\n";

}

template <> // template specialization for const std::string&

void foo::doit(const std::string& t) {

std::cout << "foo::do<std::string&> has a " << t << "\n";

}

template <> // template specialization for const int&

void foo::doit(const int& t) {

std::cout << "foo::do<int> has a " << t << "\n";

}

int main() {

foo fs;

fs.doit("what?"); // call std::string& specialization

fs.doit(3); // call int specialization

fs.doit(2.11567); // call main template function

return 0;

}

Note: generally put templated functions and classes in the header file.

See Vandervoorde and Josuttis's book on C++ templates.