Undefined reference errors from an unneeded library - c++

Question

I'm getting some "undefined reference" errors in ld and am at a loss as to what's causing them.
My makefile builds several executables using commands like this:
g++ -ogui_program1 -Lpath/to/MyLibs gui_program1.o -lMyUI -lMyBusinessLogic \
-lMyUtil -lboost_regex
g++ -ogui_program2 -Lpath/to/MyLibs gui_program2.o -lMyUI -lMyBusinessLogic \
-lMyUtil -lboost_regex
g++ -ocli_program1 -Lpath/to/MyLibs cli_program1.o -lMyUI -lMyBusinessLogic \
-lMyUtil -lboost_regex
g++ -ocli_program2 -Lpath/to/MyLibs cli_program2.o -lMyUI -lMyBusinessLogic \
-lMyUtil -lboost_regex
And so on. (Actually, there are quite a few more libraries than this, but this is the general idea.)
MyUI, MyBusinessLogic, and MyUtil are all dynamic libraries that I've already built. To make writing the makefile simpler, the same list of libraries is used for both GUI and command line programs, even though the command line programs don't need libMyUI.so.
One and only one of the command line programs is giving numerous errors about undefined references to Boost.Regex symbols when I try to link it, even though I'm linking -lboost_regex with every binary:
libMyBusinessLogic.so: undefined reference to `boost::re_detail::perl_matcher >, boost::regex_traits > >::construct_init(boost::basic_regex > > const&, boost::regex_constants::_match_flags)'
libMyBusinessLogic.so: undefined reference to `boost::cpp_regex_traits::toi(char const*&, char const*, int) const'
libMyBusinessLogic.so: undefined reference to `boost::re_detail::perl_matcher, std::allocator > >, std::allocator, std::allocator > > > >, boost::regex_traits > >::match()'
libMyBusinessLogic.so: undefined reference to `boost::re_detail::perl_matcher, std::allocator > >, std::allocator, std::allocator > > > >, boost::regex_traits > >::construct_init(boost::basic_regex > > const&, boost::regex_constants::_match_flags)'
libMyBusinessLogic.so: undefined reference to `boost::re_detail::perl_matcher, std::allocator > >, std::allocator, std::allocator > > > >, boost::regex_traits > >::find()'
libMyBusinessLogic.so: undefined reference to `boost::basic_regex > >::do_assign(char const*, char const*, unsigned int)'
libMyBusinessLogic.so: undefined reference to `boost::re_detail::perl_matcher >, boost::regex_traits > >::match()'
Linking all other programs works fine. If I remove -lMyUI from the one command-line program, then it works fine, even though MyUI doesn't show up anywhere in the error list.
Why isn't ld finding Boost.Regex symbols, when I've added -lboost_regex to the end of the command? Why does removing a seemingly unrelated library fix it? Why do other programs link without any problems?

Answer 1

I've figured out at least most of the answer. Due to some sloppiness in my makefile rules, libMyUI.so was linked against boost_regex, but libMyBusinessLogic.so wasn't. I'm guessing that, as a result, linking MyUI caused boost_regex to get pulled in prematurely, before the linker knew all of the symbols that MyBusinessLogic would need from it.
As long as I'm consistent - either all of My*.so link with boost_regex, or none of them do - everything works. I'm not sure which of these solutions is most preferred, but at least I have a fix.