Custom C++ manipulator problem

Question

Hi guys,

I'm trying to implement my own stream manipulator inside my logging class. It's basically endline manipulator which changes state of a flag. However when I try to use it, I'll get:

ftypes.cpp:57: error: no match for ‘operator<<’ in ‘log->Log::debug() << log->Log::endl’
/usr/lib/gcc/i386-redhat-linux/4.1.2/../../../../include/c++/4.1.2/bits/ostream.tcc:67: note: candidates are: std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>& (*)(std::basic_ostream<_CharT, _Traits>&)) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/i386-redhat-linux/4.1.2/../../../../include/c++/4.1.2/bits/ostream.tcc:78: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ios<_CharT, _Traits>& (*)(std::basic_ios<_CharT, _Traits>&)) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/lib/gcc/i386-redhat-linux/4.1.2/../../../../include/c++/4.1.2/bits/ostream.tcc:90: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::ios_base& (*)(std::ios_base&)) [with _CharT = char, _Traits = std::char_traits<char>]

...

Code:

class Log {
public:
  ...
  std::ostream& debug() { return log(logDEBUG); }  
  std::ostream& endl(std::ostream& out);           // manipulator
  ...
private:
  ...
  std::ofstream m_logstream;
  bool          m_newLine;
  ...
}


std::ostream& Log::endl(std::ostream& out) 
{  
  out << std::endl;
  m_newLine = true;
  return out;
}

std::ostream& Log::log(const TLogLevel level)
{
  if (level > m_logLevel) return m_nullstream;

  if (m_newLine)
  {
    m_logstream << timestamp() << "|" << logLevelString(level) << "|";
    m_newLine = false;
  }
  return m_logstream;
}

I'm getting the error when I try to call it:

log->debug() << "START - object created" << log->endl;

(log is the pointer to Log object)

Any ideas? I suspect it's somehow connected to the fact that the manipulator is actually inside the class but that's just my wild guess...

Cheers,

Tom

EDIT: Putting this here instead of comment because of limiting formatting. I tried to implement my streambuf and it works great with one exception: when I try to open filebuf for append it fails. Output works nicely, just append doesn't for some unknown reason. If I try to use ofstream directly with append it works. Any idea why? – Works:

std::ofstream test; 
test.open("somefile", std::ios_base::app); 
if (!test) throw LogIoEx("Cannon open file for logging"); 
test << "test" << std::endl;

Appends "test" correctly .

Doesn't work:

std::filebuf *fbuf = new std::filebuf(); 
if (!fbuf->open("somefile", std::ios_base::app)) throw LogIoEx("Cannon open file for logging");

Throws exception, if I set openmode to out then it works..

Cheers

Answer 1

There is defined an operator<<(ostream &, ostream &(*)(ostream&)) but not an operator<<(ostream &, ostream &(Log::*)(ostream&)). That is, the manipulator would work if it were a normal (non-member) function, but because it depends on an instance of Log, the normal overload wouldn't work.

To fix this problem, you may need to have log->endl be an instance to a helper object and, when pushed with operator<<, call the appropriate code.

Like so:

class Log {
  class ManipulationHelper {  // bad name for the class...
  public:
    typedef ostream &(Log::*ManipulatorPointer)(ostream &);

    ManipulationHelper(Log *logger, ManipulatorPointer func) :
      logger(logger),
      func(func) {
    }

    friend ostream &operator<<(ostream &stream, ManipulationHelper helper) {
        // call func on logger
        return (helper.logger)->*(helper.func)(stream);
    }

    Log *logger;
    ManipulatorPointer func;
  }

  friend class ManipulationHelper;

public:
  // ...

  ManipulationHelper endl;

private:
  // ...

  std::ostream& make_endl(std::ostream& out); // renamed
};

// ...

Log::Log(...) {
  // ...
  endl(this, make_endl) {
  // ...
}

Answer 2

That's not how manipulators work - it's all about types. What you want is something like:

class Log {
...
struct endl_tag { /* tag struct; no members */ };
static const struct endl_tag endl;
...
LogStream &debug() { /* somehow produce a LogStream type here */ }
}

LogStream &operator<<(LogStream &s, const struct endl_tag &) {
  s.m_newLine = true;
}

Note that:

1. Since m_newLine is part of Log, we can't be working with generic std::ostreams. After all, what would std::cout << Log->endl() mean? So you need to create a new stream type derived from std::ostream (I've left it out here, but assumed it's named LogStream).
2. endl doesn't actually do anything itself; all the work is in operator<<. The only purpose of it is to get the right operator<< overload to run.

That said, you're not supposed to be defining new manipulators and stream classes if you can avoid it, because it gets complex :) Can you do what you need using just std::endl, and wrapping an ostream around your own custom streambuf? That is how the C++ IO library is meant to be used.

Answer 3

Try this:

#include <iostream>

class Log
{
    public:
    class LogEndl
    {
        /*
         * A class for manipulating a stream that is associated with a log.
         */
        public:
            LogEndl(Log& p)
                :parent(p)
            {}
        private:
            friend std::ostream& operator<<(std::ostream& str,Log::LogEndl const& end);
            Log&    parent;
    };
    std::ostream& debug()   {return std::cout;}
    /*
     * You are not quite using manipulators the way they are entended.
     * But I wanted to give an example that was close to your original
     *
     * So return an object that has an operator << that knows what to do.
     * To feed back info to the Log it need to keep track of who its daddy is.
     */
    LogEndl       endl()    {return LogEndl(*this);}
    private:
        friend std::ostream& operator<<(std::ostream& str,Log::LogEndl const& end);
        bool    endOfLine;

};


std::ostream& operator<<(std::ostream& str,Log::LogEndl const& end)
{
    // Stick stuff on the stream here.
    str << std::endl;

    // Make any notes you need in the log class here.
    end.parent.endOfLine    = true;

    return str;
};

int main()
{
    Log     log;

    /*
     * Please note the use of objects rather than pointers here
     * It may help
     */
    log.debug() << "Debug " << log.endl();
}