Linker errors using static function template members of a class

Question

I don't understand why it's not compiling correctly, the linker reports missing symbols:

Undefined symbols for architecture x86_64: "bool swinadventure::Utils::is_last<__gnu_cxx::__normal_iterator > >, std::vector > >(__gnu_cxx::__normal_iterator > >, std::vector > const&)", referenced from: swinadventure::Inventory::get_item_list(std::string, std::string) in Inventory.cpp.o

Because eariler in the process I also see this error: /usr/bin/ranlib: file: liblibswinadventure.a(Utils.cpp.o) has no symbols I assume that it's not compiling the utils file correctly for some reason.

For reference, I'm using CMake on a Mac OS X 10.7 system with most homebrew tools installed and linked.

Utils.h

#ifndef UTILS_H_ #define UTILS_H_ #include <vector> namespace swinadventure { /** * Static class for various utilities and helper functions */ class Utils { public: template <typename Iter> static Iter next_iterator(Iter iter); template <typename Iter, typename Cont> static bool is_last(Iter iter, const Cont& cont); }; } /* namespace swinadventure */ #endif /* UTILS_H_ */

Utils.cpp

#include "Utils.h" namespace swinadventure { /** * Returns the next iterator * http://stackoverflow.com/questions/3516196/testing-whether-an-iterator-points-to-the-last-item * @param iter * @return */ template <typename Iter> Iter Utils::next_iterator(Iter iter) { return ++iter; } /** * Checks if the iterator is the last of the vector array * http://stackoverflow.com/questions/3516196/testing-whether-an-iterator-points-to-the-last-item * @param iter iterator * @param cont vector array * @return */ template <typename Iter, typename Cont> bool Utils::is_last(Iter iter, const Cont& cont) { return (iter != cont.end()) && (next_iterator(iter) == cont.end()); } } /* namespace swinadventure */

possible duplicate of Why can templates only be implemented in the header file? — Aaron McDaid
– Aaron McDaid, Commented Sep 15, 2012 at 18:24

Drew Dormann · Accepted Answer · 2012-09-15 16:04:09Z

Move your template function definitions to the header.

The linker is telling you that while compiling Inventory.cpp, a special instantiation of Utils::is_last was needed, but the definition wasn't available to create it.

And while compiling Utils.cpp, the definition of Utils::is_last was available, but that instantiation wasn't needed.

So neither source file was able to create that function while compiling.

Ghost2 · Accepted Answer · 2012-09-15 16:17:02Z

When you declare a template, the variables inside the <> become stand-ins for the type passed, and as such these variables are used in the same way as classes and only need to be declared once. Thus template <typename Iter, typename Cont> can should be treated as if it were a class definition, and moved outside of the class Utils { definition. The other instances of template should be removed as well, especially before the Utils::func() definitions. As it stands now, you're basically telling it "Create a class named Iter. Create a class named Iter. Create a class named Iter, etc." As soon as it encounters a variable of the type 'Iter', it doesn't know which one it should use. This site should help with templates, and is worth the look:

http://www.parashift.com/c++-faq/templates.html

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