Logging

I need some logging for my application, nothing too complicated, and I
found this (from the link cited at drdobbs)

#ifndef LOG_H
#define LOG_H

#include <stdio.h>
#include <iostream>
#include <sstream>

#define LOG(level) \
    if (level > Log::ReportingLevel()) ; \
    else Log().Get(level)

// Log, version 0.1: a simple logging class
// taken from http://www.drdobbs.com/cpp/201804215
enum TLogLevel
{
    logERROR, logWARNING, logINFO, logDEBUG, logDEBUG1, logDEBUG2, logDEBUG3, logDEBUG4
};

class Log
{
private:
    Log(const Log&);
    Log& operator =(const Log&);

    TLogLevel messageLevel;

public:
    Log() {}
    virtual ~Log();
    std::ostringstream& Get(TLogLevel level = logINFO);

public:
    static TLogLevel& ReportingLevel();

protected:
    std::ostringstream os;
};

std::ostringstream& Log::Get(TLogLevel level)
{
    os << std::string(level > logDEBUG ? 0 : level - logDEBUG, '\t');
    messageLevel = level;
    return os;
}

Log::~Log()
{
    os << std::endl;
    fprintf(stderr, "%s", os.str().c_str());
    fflush(stderr);
}

#endif /* LOG_H */

which looks perfectly fine for me, BUT this thing is puzzling
    static TLogLevel& ReportingLevel();

to actually use this code I should

int main() {
    Log::ReportingLevel() = logDEBUG1;
    LOG(logWARNING) << "warning";
}

I get the infamous not found symbol
Undefined symbols:
  "Log::ReportingLevel()", referenced from:
      testLogging() in test.o
      testLogging() in test.o
ld: symbol(s) not found
collect2: ld returned 1 exit status

Now a couple of questions
- what can be the problem? (Log.hpp was an header included and that
  thing is public)

- is assigning the value of a function in that way good? Doesn't really
  look that nice to me...)

- what is the sense of having multiple "public" / "private".
  I've seen it in other code also but I don't get why someone should do that...