C++ Log File Class (Forget Debuggers!)

C++ can become very difficult to use especially with large projects where you can't see all the variables. Here's a very simple C++ logging class that can take logs for your software without any effort. This will make you a better programmer as you won't have to rely on debuggers like in Microsoft's Visual Studio.


First you should create a project, with files like log.h and log.cpp. Let's show you the log.h file:

#include <fstream>

using namespace std;

class Log {
    Log(char* filename);
    void Write(char* logline);
    ofstream m_stream;

I included fstream for ofstream (which is an output file stream).

The logging class is called Log, it has one private (non-accessible variable by non-class-members) member called m_stream which will be the file stream to write our logs.

I have 3 functions, one constructor called Log, which takes a char pointer input filename, and a destructor ~Log to close our file and avoid memory leaks, and finally a void Write function which will take a logline char pointer.

Here's the log.cpp file:

#include "log.h"

Log::Log(char* filename) {

void Log::Write(char* logline){
  m_stream << logline << endl;


I included the log.h, then defined the implementation for the 3 functions.

The constructor will open the file delivered by filename on our stream.

The Write function will take a string and write it to the log using << operator, and will add a new line (endl) to the end of each call.

~Log the destructor will close the stream.

This really helps with debugging, so now I'll demonstrate how it can be used in a project. Of course the code can be improved so that you write less code in your main project.

#include "log.h"
#include <cstdlib>

int main(){
  Log *pLog = new Log("errors.log");
  pLog->Write("Going into our loop");
  for(int i = 0; i < 10; i++){
    char c[50];
    sprintf(c, "Looped: %d times", i);
  return 0;

I created a Log class pointer, with the parameter "errors.log" which will be my error logs file.

I call Write to write a line of errors to my log.

If you need to include integers or other variable types, it may be an idea to use sprintf to add the integer i to a char, which you can then write to your log.

Variadic Functions

If you want to avoid using sprintf, and just using Write with va_list, or variadic argument lists:

pLog->Write("Looped: %d times!", i); // no sprintf needed!

Using the following modification to Write function, you can create a C++ variadic function. This should allow you to avoid using sprintf every time.

#include <stdarg.h>

void Log::Write(const char* logline, ...){
  va_list argList;
  char cbuffer[1024];
  va_start(argList, logline);
  vsnprintf(cbuffer, 1024, logline, argList);
  m_stream << cbuffer << endl;

You can also modify it further, by adding time stamps, that will be an Inferno Development Programming Challenge on our Programming Forums.

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