Question

i would like to create a flexible logger class. I want it to be able to output data to a file or to standard output. Also, i want to use streams. The class should look something like:

class Logger
{
private:
   std::ostream m_out; // or ofstream, iostream? i don't know
public:

   void useFile( std::string fname);
   void useStdOut();

   void log( symbol_id si, int val );
   void log( symbol_id si, std::string str );
   //etc..
};

The symbol_id is an enum and defines the formatting. What i want to achieve is to be able to easily switch from standart output to a file and vice versa (this is the purpose of the use* methods). Preferably by just using m_out and simply writing m_out << "something"; without any checks whether i want to write to a file or stdout.

I know there are many ways how to get around this (using if's to test if i want to write to a file or stdout, the “C way” (using FILE* and fprintf)) and so on, but i’m sure there must be a way how to achieve this with C++ streams in a nice way. But i can’t seem to find the way how to do it. Can somebody help me please?

Answer 1

The way I’ve attacked this problem before is to make Logger an abstract base class and create separate FileLogger and OutStreamLogger classes. Then create a CompositeLogger object that implements the Logger interface, which just outputs all loggers:

CompositeLogger compLogger;
compLogger.Add(new FileLogger("output.txt"));
compLogger.Add(new StreamLogger(std::cout));
...
compLogger.log(...);

If you don’t need this level of flexibility and want to keep all this in a single class you could make the m_Out variable a pointer to std::ostream and add an extra flag to keep track of whether you need to delete it on cleanup:

private:
  std::ostream*   m_out;
  bool            m_OwnsStream;

Logger() {
   m_Out=&std::cout; // defaults to stdout
   m_OwnsStream=false;
}
void useFile(std::string filename) {
  m_Out=new std::ofstream(filename);
  m_OwnsStream=true;
}
~Logger() {
  if (m_OwnStream) 
    delete m_Out; 
}

Obviously you’d need some more checks in useFile and useStdOut to prevent memory leaks.