On MSVC (latest) the following code yields a compile error with flat_map, but not with std::map. I do not have the knowledge of std::less<> to resolve what causes the error. auto fm = boost::container::flat_map<std::string, int, std::less<> >{}; auto m = std::map<std::string, int, std::less<> >{}; auto k = std::string_view{ "my_key" }; auto x = m.find(k); // Compiles auto y = fm.find(k); // Does NOT compile Error message: 1>boost\container\flat_map.hpp(1314): error C2664: 'boost::container::vec_iterator<boost::container::dtl::pair<Key,T> *,true> boost::container::dtl::flat_tree<boost::container::dtl::pair<Key,T>,boost::container::dtl::select1st<Key>,Compare,boost::container::new_allocator<T2>>::find(const std::basic_string<char,std::char_traits<char>,std::allocator<char>> &) const': cannot convert argument 1 from 'const std::string_view' to 'const std::basic_string<char,std::char_traits<char>,std::allocator<char>> &' 1> with 1> [ 1> Key=std::string, 1> T=int, 1> Compare=std::less<void>, 1> T2=boost::container::dtl::pair<std::string,int> 1> ] 1> boost\container\flat_map.hpp(1314): note: Reason: cannot convert from 'const std::string_view' to 'const std::basic_string<char,std::char_traits<char>,std::allocator<char>>' 1> boost\container\flat_map.hpp(1314): note: No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called Best regards /Viktor