Constructors and destructors, behaving unexpectedly

[tomcr]

I have the following constructor for a class:

Cdata::Cdata(char *input)
{
    //Try and open the input file
    ifstream ifile(input, ios::in);
    if(!ifile.is_open())
    {
         cout << &quot;The input file didn't open!&quot; << endl;
         exit(1);
    }
}

When I call the constructor to open a file for the class it opens the file and goes through all the lines happily, it doesn't enter the if statement. However the minute the constructor has finished I find that the file has been closed. I'm not sure why, I've checked with my debugger and the code does not enter the destructor, nor am I trying to use the class outside of the scope I declared it in so I can't figure out what is going on. Why is the file being closed after the constructor has finished?

 

[palbano]

> the minute the constructor has finished I find that the file has been closed.

Yes, that is because your file stream object is local to the constructor:

ifstream ifile(..);

Therefore it's destructor runs when your constructor finishes thereby closing the file. If you want the file to remain open you need to make the stream object a member of your class, i.e.:

class CData{
protected:
ifstream* _ifs;
}

Cdata::Cdata(char *input)
{
//Try and open the input file
_ifs = new ifstream(input, ios::in);
if(!ifs || !_ifs->is_open())
{
cout << &quot;The input file didn't open!&quot; << endl;
exit(1);
}
}
// destructor needs to delete the input stream object which
// in turn will close the file
Cdata::~Cdata(){
delete _ifs;
}

Hope this helps
-pete

 

[tomcr]

Thanks very much!

I had ifile declared as a private (ifstream) member of the class anyway but I guess because I used the line

ifstream ifile(input, ios::in);

It overloaded the variable (so it was referring to a local ifile) or something and like you said invoked the destructor when it went out of scope.
I now simply use:

ifile.open(input, ios::in);

So that it refers directly to ifile as a member of the class, and now it all works.

Great.