C++ class redefinition error

Question

I am compiling a logging program, but I am receiving this error and cant figure it out for the life of me...

logger.cpp:15: error: redefinition of ‘class Logger’
logger.h:20: error: previous definition of ‘class Logger’

with gcc when i compile with

g++ -Wall logger.cpp -o log

logger.h:

#ifndef LOGGER_H
#define LOGGER_H

#include <fstream>
#include <iostream>
#include <string>
using std::string;

class Logger
{

static Logger* m_pInstance;

public:
static Logger* Instance() { return m_pInstance; }
void writeLog(string message);
void openLogFile(string fileName);
void closeLogFile();
void deleteLogger();

};
#endif

logger.cpp

#include "logger.h"

#include <fstream>
#include <iostream>

class Logger
{
static Logger* m_pInstance;
std::ofstream m_pOutputFile;
Logger()
{
}
~Logger()
{
}

public:
static Logger* Instance()
{
    if(!m_pInstance)
    {
        m_pInstance = new Logger;
    }
    return m_pInstance;
}
void writeLog(std::string message)
{
    m_pOutputFile << message << "\n";
    std::cout << "you just wrote  " << message << "  to the log file!\n" << std::endl;
}
void openLogFile(std::string fileName)
{
    m_pOutputFile.open(fileName.c_str(),std::ios::out);
}
void closeLogFile()
{
    m_pOutputFile.close();
}
void deleteLogger()
{
    delete m_pInstance;
}
};
Logger* Logger::m_pInstance = NULL;

Answer 1

Compiler always expects only one class definition in the whole namespace(or scope) that class belongs to. Currently in the code you specified, you would see that there are infact 2 class definitions: one in .h file and another one in .cpp file. That is why the compiler is complaining that you are redefining a class which is not allowed.

Generally whenever you encounter a compiler error, it's a good idea to look at the lines that compiler tells. Most of the time the problem is in the line the compiler points out.

Answer 2

It's exactly what the error message says. The implementation file can't just provide a redefinition of the class adding new member variables and conflicting function bodies wherever it pleases. Instead, provide definitions for the functions and static member variables you've already declared.

#include "logger.h"

#include <fstream>
#include <iostream>


static Logger::Logger* m_pInstance;

Logger::Logger()
{
}

Logger::~Logger()
{
}

// this also is illegal, there's a body provided in the header file
//Logger* Logger::Instance()
//{
//    if(!m_pInstance)
//    {
//        m_pInstance = new Logger;
//    }
//    return m_pInstance;
//}

void Logger::writeLog(std::string message)
{
    m_pOutputFile << message << "\n";
    std::cout << "you just wrote  " << message << "  to the log file!\n" << std::endl;
}

and so on

Answer 3

Well, because you are redefining the class. You can't say 'class Logger {' again in the .cpp when you already included it from the .h.