Can #define preprocessor directive contain if and else?

Question

I was trying the logger code from this link, but it gives me error. How to implement a good debug/logging feature in a project

#ifndef _LOGGER_HPP_
#define _LOGGER_HPP_

#include <iostream>
#include <sstream>

/* consider adding boost thread id since we'll want to know whose writting and
 * won't want to repeat it for every single call */

/* consider adding policy class to allow users to redirect logging to specific
 * files via the command line
 */

enum loglevel_e
    {logERROR, logWARNING, logINFO, logDEBUG, logDEBUG1, logDEBUG2, logDEBUG3, logDEBUG4};

class logIt
{
public:
    logIt(loglevel_e _loglevel = logERROR) {
        _buffer << _loglevel << " :" 
            << std::string(
                _loglevel > logDEBUG 
                ? (_loglevel - logDEBUG) * 4 
                : 1
                , ' ');
    }

    template <typename T>
    logIt & operator<<(T const & value)
    {
        _buffer << value;
        return *this;
    }

    ~logIt()
    {
        _buffer << std::endl;
        // This is atomic according to the POSIX standard
        // http://www.gnu.org/s/libc/manual/html_node/Streams-and-Threads.html
        std::cerr << _buffer.str();
    }

private:
    std::ostringstream _buffer;
};

extern loglevel_e loglevel;

#define log(level) \
if (level > loglevel) ; \
else logIt(level)

#endif

More precisely, this #define is giving errors:

#define log(level) \
if (level > loglevel) ; \
else logIt(level)

The errors are Syntax error: if and Syntax error: else

But later, I noticed that if I move #include "logger.hpp" from main.h to main.cpp, the problem disappeared. Though 'main.h' is included many times in different places, it does contain '#pragma once'.

Any idea?

Answer 1

If loglevel is known at compile time you can do the following:

template <bool>
struct LogSystem
{
    template <class T>
    LogSystem& operator << (const T &)
    {
        //ignore the input
        return (*this);
    }
};

template <>
struct LogSystem <true>
{
    template <class T>
    LogSystem& operator << (const T & v)
    {
        cout << v;
        return (*this);
    }
};

template <bool B>
LogSystem<B>    getLog()
{
    return LogSystem<B>();
}

#define log(level) getLog< (level <= loglevel) >()

if loglevel is not known at compile time:

class iLogSystem
{
public:
    virtual iLogSystem& operator << (const int &)
    {
        //empty
        return (*this);
    }
    virtual iLogSystem& operator << (const custom_type &);
    {
        return (*this);
    }
    //make functions for logging all the types you want
};

class LogSystem : public iLogSystem
{
public:
    virtual iLogSystem& operator << (const int & v)
    {
        cout << v;
        return (*this);
    }
    virtual iLogSystem& operator << (const custom_type &  q);
    {
        cout << q.toString();
        return (*this);
    }
    //make functions for logging all the types you want
};
iLogSystem& getLog(const bool l)
{
    static LogSystem actual_log;
    static iLogSystem empty_log;
    if(l)
        return &actual_log;
    return &empty_log;
}

#define log(level) getLog( level <= loglevel )