Linker error: undefined reference to `Reference_Genome::seq[abi:cxx11]' - c++

Question

I am pretty new to c and c++, so please try explain more specific what I should do. The program tries to read files from a directory using multithreads, and store the information in a map so that it can be used later.
I have been looking for similar posts. However, I am not able to figure out.
In https://github.com/kaldi-asr/kaldi/issues/938, it said that "If you get linker errors about undefined references to symbols that involve types in the std::__cxx11 namespace or the tag [abi:cxx11] then it probably indicates that you are trying to link together object files that were compiled with different values for the _GLIBCXX_USE_CXX11_ABI macro."
The solution for undefined reference to `pthread_cancel' (add "-pthread" flag does not work either.
My code is
#include <iostream>
#include <iomanip>
#include <fstream>
#include <vector>
#include <map>
#include <algorithm>
#include <random>
#include <unistd.h>
#include <cmath>
#include <stdlib.h>
#include <mutex>
#include <sys/wait.h>
#include <filesystem>
#include <string>
#include <pthread.h>
#define time_str(s) (s < 60 ? (to_string(s) + " second(s)") : (s < 3600 ? (to_string((s) / 60) + " minute(s)") : (to_string((s) / 3600) + " hour(s) and " + to_string(((s) % 3600) / 60) + " minute(s)")))
using namespace std;
namespace fs = std::filesystem;
struct MyGenom
{
vector<string> filepaths;
map<string, string> seq;
};
void check_rv(int rv) {
if (rv != 0) {
printf("Error: Value is %d\n", rv);
exit(1);
}
}
struct Reference_Genome {
static long unsigned int idx;
static map <string, string> seq;
static pthread_mutex_t mtxLock;
static vector <string> filepaths;
static void writing(string path) {
}
static void *distribution(void *var) {
}
Reference_Genome(string dir, unsigned int n_threads) {
}
};
int main(int argc, char const *argv[]) {
string dir = "./data/ex_seq";
unsigned int n_threads = 5;
Reference_Genome ref(dir, n_threads);
cout << "chr6: " << ref.seq["chr6"] << endl;
cout << "chr9: " << ref.seq["chr9"] << endl;
cout << "chr13: " << ref.seq["chr13"] << endl;
}
The gcc version is "Thread model: posix
gcc version 9.3.0 (Ubuntu 9.3.0-10ubuntu2)".
The error is
testSeq.cpp:97: undefined reference to `Reference_Genome::seq[abi:cxx11]'
/usr/bin/ld: testSeq.cpp:98: undefined reference to `Reference_Genome::seq[abi:cxx11]'
/usr/bin/ld: testSeq.cpp:99: undefined reference to `Reference_Genome::seq[abi:cxx11]'
/usr/bin/ld: /tmp/cctfwVX2.o: in function `Reference_Genome::writing(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >)':
/testSeq.cpp:46: undefined reference to `Reference_Genome::seq[abi:cxx11]'
/usr/bin/ld: testSeq.cpp:48: undefined reference to `Reference_Genome::seq[abi:cxx11]'
/usr/bin/ld: /tmp/cctfwVX2.o: in function `Reference_Genome::distribution(void*)':
testSeq.cpp:55: undefined reference to `Reference_Genome::filepaths[abi:cxx11]'
/usr/bin/ld: testSeq.cpp:55: undefined reference to `Reference_Genome::idx'
/usr/bin/ld: testSeq.cpp:56: undefined reference to `Reference_Genome::mtxLock'
/usr/bin/ld: testSeq.cpp:57: undefined reference to `Reference_Genome::idx'
/usr/bin/ld: testSeq.cpp:57: undefined reference to `Reference_Genome::filepaths[abi:cxx11]'
/usr/bin/ld: testSeq.cpp:58: undefined reference to `Reference_Genome::idx'
/usr/bin/ld: testSeq.cpp:58: undefined reference to `Reference_Genome::idx'
/usr/bin/ld: testSeq.cpp:59: undefined reference to `Reference_Genome::mtxLock'
/usr/bin/ld: /tmp/cctfwVX2.o: in function `Reference_Genome::Reference_Genome(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, unsigned int)':
testSeq.cpp:68: undefined reference to `Reference_Genome::filepaths[abi:cxx11]'
/usr/bin/ld: testSeq.cpp:70: undefined reference to `Reference_Genome::idx'
/usr/bin/ld: testSeq.cpp:72: undefined reference to `Reference_Genome::mtxLock'
/usr/bin/ld: testSeq.cpp:85: undefined reference to `Reference_Genome::mtxLock'
/usr/bin/ld: testSeq.cpp:88: undefined reference to `Reference_Genome::filepaths[abi:cxx11]'
collect2: error: ld returned 1 exit status

Answer 1

When you declare static variables inside a class, you must also declare it exactly once outside of the class. In this case, you could put this in the bottom of your C++ file or in between the main() function and the class Reference_Genome definition:
long unsigned int Reference_Genome::idx;
map <string, string> Reference_Genome::seq;
pthread_mutex_t Reference_Genome::mtxLock;
vector <string> Reference_Genome::filepaths;
The idea is that you can put the class definition inside a header file, to be included in multiple different compilation units, but the static variables are only defined once, in one .cpp file of your choosing.