Link Error when use variables_map::contains method

[YunchengLiu]

Env: Windows 10 19044.1826
Library Config: vcpkg, all ports are updated to date.
IDE: Visual Studio 2022 17.2.6
Boost Version: 1.79.0
C++ Version: C++20

When i use program_options library in my program, i got a link error as follows.

main.obj : error LNK2019: unresolved external symbol "__declspec(dllimport) public: bool __cdecl std::_Tree<class std::_Tmap_traits<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> >,class boost::program_options::variable_value,struct std::less<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > >,class std::allocator<struct std::pair<class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const ,class boost::program_options::variable_value > >,0> >::contains(class std::basic_string<char,struct std::char_traits<char>,class std::allocator<char> > const &)const " (__imp_?contains@?$_Tree@ V?$_Tmap_traits@V?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@std@@Vvariable_value@program_options@boost@@U?$less@V?$basic_string@ DU?$char_traits@D@std@@V?$allocator@D@2@@std@@@2@V?$allocator@U?$pair@$$CBV?$basic_string@DU?$char_traits@D@ std@@V?$allocator@D@2@@std@@Vvariable_value@program_options@boost@@@std @@@2@$0A@@std@@@std@@QEBA_NAEBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@2@@Z), function "__cdecl This is referenced in exec(class boost::program_options::options_description const &,class boost::program_options::variables_map const &)" (?exec@@YA@AEBVoptions_description@program_options@boost@@AEBVvariables_map@23@@Z) symbol

Other boost libraries are work well with my vcpkg and VS env.

The reproduce code:

#include <cstdlib>
#include <iostream>
#include <boost/program_options.hpp>

namespace po = boost::program_options;

auto initDesc(po::options_description& desc) {

    desc.add_options()
        ("--help", "Produce help message")
        ;

}

auto exec(const po::options_description& desc, const po::variables_map& vm) {
    // contains method will cause a link error
    // but count method will perform normal
   // Code is compiled with an 'std=c++20' option
    if (vm.contains("--help")) {
        std::cout << desc << std::endl;
        return EXIT_FAILURE;
    }

    return EXIT_SUCCESS;

}

int main(int argc, char* argv[]) {

    try {

        po::options_description opts{ "All Supported Options" };
        po::variables_map vm;

        initDesc(opts);

        po::store(po::parse_command_line(argc, argv, opts), vm);
        po::notify(vm);

        return exec(opts, vm);

    }
    catch (const std::exception& e) {
        std::cerr << e.what() << std::endl;
    }

    return EXIT_SUCCESS;

}

I noticed that variables_map are inherited from std::map, is something wrong with here?

[aleksmurfitt]

This is quite an old issue, but you're getting this error because your Boost libraries aren't targeting C++20.

As you correctly noted variables_map inherits from std::map, so while your compiler is happy to use C++20 methods like std::map::contains, if Boost isn't being built for C++20 then the symbols won't be available at link time. I would look into how vcpkg builds Boost - it's almost certainly targeting an older standard.

In the meantime, vm.find("--help") != vm.end() will work just fine and is exactly the same as vm.contains("--help"), albeit less convenient.