error

apitest.o: In function `get_database(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, void (*)(Xapian::WritableDatabase&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&), std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)':
/home/ayush/Desktop/xapian-patch/xapian/xapian-letor/tests/apitest.cc:62: undefined reference to `BackendManager::get_database(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, void (*)(Xapian::WritableDatabase&, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&), std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
harness/backendmanager_multi.o:(.rodata._ZTI19BackendManagerMulti[_ZTI19BackendManagerMulti]+0x10): undefined reference to `typeinfo for BackendManager'
harness/backendmanager_multi.o:(.rodata._ZTV19BackendManagerMulti[_ZTV19BackendManagerMulti]+0x40): undefined reference to `BackendManager::get_writable_database_path(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
harness/backendmanager_multi.o:(.rodata._ZTV19BackendManagerMulti[_ZTV19BackendManagerMulti]+0x48): undefined reference to `BackendManager::get_remote_database(std::vector<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > const&, unsigned int)'
harness/backendmanager_multi.o:(.rodata._ZTV19BackendManagerMulti[_ZTV19BackendManagerMulti]+0x50): undefined reference to `BackendManager::get_writable_database_as_database()'
harness/backendmanager_multi.o:(.rodata._ZTV19BackendManagerMulti[_ZTV19BackendManagerMulti]+0x58): undefined reference to `BackendManager::get_writable_database_again()'
collect2: error: ld returned 1 exit status
Makefile:506: recipe for target 'apitest' failed
make: *** [apitest] Error 1