Thread: Writting to file in callback function

Hey everyone! I want to do a basic thing but for some strange reason there is something not working.

I need to write to a file but I'm handling all the code in a callback function, which in turn is declared in a class. Here is the architecture of my code:

Code:

int main (int argc, char **argv)
{
  
  ImageConverter Aclass;
  while(1); //infinite loop
  return 0;
}

class ImageConverter
{
//some variables ..
  
public:

ImageConverter ():
 
  {

   

  }

  ~ImageConverter ()
  {

  }

  void imageCb () //this function gets called when I receive an image frm server
  {

    .... //some calculations
    
    fstream outfile ("directory/train.txt");
     
    outfile.open ("directory/train.txt", fstream::in | fstream::out | fstream::app);
    
    outfile<<"P,"<<SomeValues<<endl;
    
    outfile.close();
   
   ....


  }

};

But this doesn't work, the file gets created but it allways overwrites itself which is not what I need to do.

Can anyone please tell me what I am doing wrong? Is it because i am constantly declaring outfile?

Thanks

From the information you have provided, it appears that ImageConverter()::imageCB() is never called. Which means the file will never be created, let alone "overwrite itself".

Accordingly, the code will never produce output. All your main() function will do is invoke the constructor for ImageConverter, which does nothing, and then enter an infinite loop.

Try providing a small sample of compilable code that actually exhibits your problem.

More generally, read this link to learn how to ask questions that have a chance of eliciting a useful response.

>>the file gets created but it allways overwrites itself which is not what I need to do.
Everytime you get to this line

fstream outfile ("directory/train.txt");

you overwrite the file (ie truncate it).
Perhaps you intend for this to be in main, in order to truncate the file on start, and then subsequently open it for appending?

Btw, it is better if you open the file once instead of opening and closing it all the time. It might be better to open the file in main then pass it to your imageconverted so it can write to it.
Also, files close automatically when they go out of scope. No need to manually call close.

Originally Posted by rags_to_riches

I don't know what the consequence of calling open on this object after the constructor has been called in the previous way, might be undefined, might reopen with the new settings, but by that time your file has been blown away.

According to cplusplus.com, it will fail if you reopen it after a file has already been opened.

fstream's constructor does not open with the app modifier by default. It is in | out, which will truncate an existing file.

@ grumpy:

I'm sorry I wasn't to clear in my OP. I'm programming under ROS (Robot operating system) environment, which has it's own funcionalities and I didn't want to put to much information that would distract you from my main problem. I'm creating a software application for visual detection of humans in images of urban setting. The code is kinda complex and uses severel files for compilation. For you to compile it you would need to download ROS, and that is not the point.

I have two programs running at the same time. One is the server, that is feeding the client with images every x second. Everytime an image arrives the client, ImageCb function gets called and I need to write some information to a file about it.

@ Elysia

I thought about creating the file on main and then opening it on the callback function, but I have no clue of how to do that. I tried to declare fstream on the ImageConverter constructor but even then I can't open it in the ImageCb. Keep getting "outfile isn't declared in this scope" error. How do I pass that information from the main to the class and then to the ImageCb?

That is actually not what I meant you should do. I meant something like:

#include <fstream>

Code:

class ImageConverter
{
private:
	std::fstream& m_file;
public:
	ImageConverter(std::fstream& file): m_file(file) {}
	void ImageCb()
	{
		m_file << whatever;
	}
};

int main()
{
	std::fstream file("C:\\test.txt");
	ImageConverter converter(file);
}

It is possible the way I did it, without any smart pointers. Which way is more straight forward is subjective.
However, I find no information that speculates on whether you can open a file that is already open. This, more than likely, is platform dependent. That is, your code may or may not work, so I will not recommend it.