How to use static ofstream correctly in C++11?

For this program, I want to use Singleton Design Pattern. This program is doing a logger particularly. When one instance is created, it first opens one file. Then when public method report(const string&) invoked, it appends the string to the end of the file. When the program exits, the stream should be closed.

Here are my code:

Logger.h

class Logger{

public:
    static Logger& instance(){
        static Logger log;
        return log;
    }
    void report(const string&);

private:
    Logger();
    Logger(const Logger&){}
    Logger& operator=(const Logger&){}
    static void clean();
    std::ofstream logFile_;
};

main.cpp

#include <iostream>
#include <fstream>
using std::string;
#include "Logger.h"

using std::cout; using std::endl;

void Logger::clean(){
    //    logFile_.close();
};

Logger::Logger(){
    logFile_ = std::ofstream("log.txt", std::ios::out);
    std::atexit(&clean);
}

void Logger::report(const string& comment){
    //    logFile_ << comment;
    //    logFile_.flush();
}

int main() {

    string comment = "Initialization";

    Logger::instance().report("");

    Logger& logger1 = Logger::instance();
    logger1.report(comment);

    Logger& logger2 = Logger::instance();
    logger2.report(comment);

    return 0;
}

For my purpose, I should use static ofstream in function clean() in this case, so I changed by logFile declaration to static std::ofstream logFile_, but this gives an link error:

Undefined symbols for architecture x86_64: "Logger::logFile_", referenced from: Logger::Logger() in main-158c70.o ld: symbol(s) not found for architecture x86_64 clang: error: linker command failed with exit code 1 (use -v to see invocation)

My platform is Mac OS 10.12, compile command is Clang++ -std=c++11 main.cpp

Is there any possible solution to this problem? Or is there another way to do the coding?