RunOrVeith RunOrVeith - 1 year ago 85
C++ Question

What's wrong with my file logger class in C++?

I'm trying to summarize some experimental results in a file.
The results are generated in various C++ classes and files.
I want them to all write to the same file.

For this reason it would be conventient to have a header that defines everything, and then I can just include it in the files that need to write to it. I should be a singleton, so it is not tried to open the same file twice.

It looks like this:


#include <fstream>

class FileLogger{
std::ofstream *logfile;
static FileLogger *s_instance;


logfile = new std::ofstream();
logfile->open("~/results/experiments.txt", std::ios_base::app);

std::cout << "Destructor of logger called" << std::endl;
delete logfile;
delete s_instance;

static std::ofstream *instance()
if (!s_instance)
s_instance = new FileLogger();

std::cout << "got logger" << std::endl;

return s_instance->logfile;


FileLogger *FileLogger::s_instance = 0;

#endif // FILELOGGER_H

I would now think that in another file I just do:

#include "FileLogger.h"

and then use

*FileLogger::instance() << "Testoutput" << std::endl;

to write to the file. However, if I try it out, the file is not created; if I create it per hand nothing is written to it.
I do get the output of "got logger", that is called when the logger is accessed via the instance method.
I also noticed that the destructor is never called.

Why is this not working / Is this bad style?

Answer Source

This code

    logfile->open("~/results/experiments.txt", std::ios_base::app);

tries to open a file with the literal name ~/results/experiments.txt. Tilde expansion to your home directory is done by your command shell (probably bash). You need to use the actual name of your home directory, for example:

    logfile->open("/home/yourusername/results/experiments.txt", std::ios_base::app);
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