Managing a log stream in C++ in a cout-like notation

Question

I have a class in c++ in order to write log files for an application of mine. I have already built the class and it works, it is something like this:

class Logger {
   std::string _filename;
public: 
   void print(std::string tobeprinted);
}

Well, it is intuitive that, in order to print a line in the log file, for an object of Logger, it is simply necessary to do the following:

Logger mylogger("myfile.log");
mylogger.print(std::string("This is a log line"));

Well. Using a method approach is not the same as using a much better pattern like << is. I would like to do the following:

Logger mylogger("myfile.log");
mylogger << "This is a log line";

That's all. I suppose I must overload the << operator... But overloading using this signature (the classic one):

ostream& operator<<(ostream& output, const MyObj& o);

But I do not have a ostream... So, should I do as follows?

Logger& operator<<(Logger& output, const std::string& o);

Is this the right way? Thanks

Answer 1

class Log
{
public:

    enum Level { Debug, Error, Info };

    static ostream& GetStream() { return cout; }
    static bool IsLevelActive(Level l) { return true; }
};

#ifndef NO_LOG
#define LOG_ERROR(M)   do { if (Log::IsLevelActive(Log::Error))   (Log::GetStream() << "ERR: " << M << "\n"); } while (false)
#define LOG_INFO(M)    do { if (Log::IsLevelActive(Log::Info))    (Log::GetStream() << "INF: " << M << "\n"); } while (false)
#define LOG_WARNING(M) do { if (Log::IsLevelActive(Log::Warning)) (Log::GetStream() << "WRN: " << M << "\n"); } while (false)
#else
#define LOG_ERROR(M)
#define LOG_INFO(M)
#define LOG_WARNING(M)
#endif

struct MyObject {
    int a, b;
};

ostream& operator<<(ostream& ostr, const MyObject& obj) {
    ostr << "(a=" << obj.a << ", b=" << obj.b << ")";
    return ostr;
}

void test() {
    int v1 = 42;
    int v2 = 43;
    LOG_INFO("value1=" << v1 << ", value2=" << v2);

    MyObject o = {1, 2};
    LOG_INFO("obj=" << o);
}