c++ Singleton error : undefined reference to singleton::instance [duplicate]

I have logger class with singleton instance and main function

// Logger.h
class Logger
{

public:
    enum LogLevel { ALL, INFO, WARNING, ERROR, NEEDED };

    static std::shared_ptr<Logger> getInstance()
    {
        if (instance == 0)
        {
            instance.reset(new Logger());
        }
        return instance;
    }

    Logger(LogLevel level=LogLevel::WARNING)
    {
        Errorlevel = level;
        if (Errorlevel <= INFO)
        {
            cout << "[" << LogLevelNames[Errorlevel] << "]: " << "LOGGER set to: " << LogLevelNames[Errorlevel] << std::endl;
        }
    }

    ~Logger(){}
private:
    LogLevel Errorlevel = ALL;
    string LogLevelNames[5] = { "ALL","INFO","WARNING", "ERROR", "NEEDED" };
    static std::shared_ptr<Logger> instance;

};

using above Logger singleton in main function

// main.cpp
int main(int argc, char **argv)
{
  std::shared_ptr<Logger> logger = std::make_shared<Logger> (Logger::LogLevel::WARNING);

  if(ip_address == "UNKNOWN" && admin == "UNKNOWN")
  {
     logger->log(Logger::ERROR, "Check config for cam_ip_address or cam_admin");
     return (0);
  }

}

Getting following error:

main.cpp:(.text._ZN5CFaceC2ESt6vectorINSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEESaIS6_EES6_[_ZN5CFaceC5ESt6vectorINSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEESaIS6_EES6_]+0x166): undefined reference to `Logger::instance'

how to resolve this issue ?