Question

I am having a strange linking error. I followed instructions presented here to avoid this kind of problems, but I can’t figure out how to split headers and implementation files.

Here’s my test file:

#include <cargo.h>
#include <iostream>
#include <string>

using namespace std;
using namespace dom;

int main()
{
    dom::cargo<string> text("Hello, World!");

    cout << text << endl;

    return 0;
}

Header file for class cargo included in the test:

#ifndef CARGO_H
#define CARGO_H 1

#include "node.h"

#include <iostream>

namespace dom
{
    template <typename Type> class cargo;

    template <typename Type>
    std::ostream& operator<<(std::ostream&, const dom::cargo<Type>&);

    template <typename Type>
    class cargo
        : public dom::node
    {
        Type value;

    public:
        cargo()
            : value()
        { }

        cargo(Type value)
            : value(value)
        { }

        friend std::ostream& operator<< <>(std::ostream&, const dom::cargo<Type>&);
    };
}

#endif // !CARGO_H

And its implementation:

#include "cargo.h"

template <typename Type>
std::ostream& operator<< (std::ostream& ostream, dom::cargo<Type>& cargo)
{
    return (ostream << cargo.value);
}

I’m using CMake to compile and link it all togather.
The linking error I get is about undefined reference to operator <<:

Scanning dependencies of target test
[100%] Building CXX object Tests/CMakeFiles/test.dir/test0.c++.o
Linking CXX executable test
CMakeFiles/test.dir/test0.c++.o: In function `main':
test0.c++:(.text+0x9b): undefined reference to `std::basic_ostream<char, std::char_traits<char> >& dom::operator<< <std::basic_string<char, std::char_traits<char>, std::allocator<char> > >(std::basic_ostream<char, std::char_traits<char> >&, dom::cargo<std::basic_string<char, std::char_traits<char>, std::allocator<char> > > const&)'
collect2: ld returned 1 exit status
make[2]: *** [Tests/test] Error 1
make[1]: *** [Tests/CMakeFiles/test.dir/all] Error 2
make: *** [all] Error 2

What am I doing wrong? Please help me!

Answer 1

(Member) function templates are not functions; the linker doesn’t see them unless you instantiate them. Source files are compiled separately, so if you put a (member) function template in one source file and don’t explicitly instantiate it, the linker won’t see it.

So in your case, the function template has not been turned into a function in cargo.o, so the linker reports an error because main.o depends on it. You need to put the template in the header file, or explicitly instantiate it in cargo.cpp.