A memory leak (I think...)

Hi,

I'm writing a movie database program. One of the features I want to include is the ability to import a list of movie titles from a text file. The Import function reads in all the titles from a filename of the user's choosing, puts those titles into a list, and then copies that list into another sorted list of movie titles. Here's my code:

Code:

---

void Import(MovieList& ToList, String FromFile)
{
        ifstream inFile;                //input file stream inFile
        inFile.open(FromFile);//open inFile with user-specified filename

        MovieList FromList;        //list to hold titles from file
        String ImportedTitle;        //a title from the file

        while (inFile)        //while not at the end of the file
        {
                //read in one line of the file as a string
                inFile.getline(ImportedTitle, 40);       

                //add that title from the file into a list
                Add(FromList, ImportedTitle);
        }

        inFile.close();        //close the file

        //the head of the file's title strings
        Movie* FromLocation = FromList.Head;

        //while the title list from the file has not ended
        while (FromLocation != NULL)
        {
                //add title from file list into the database
                Add(ToList, FromLocation->Title);

                //go to next title from file
                FromLocation = FromLocation->Next;
        }
}

---

When I run this function in my program, I get a runtime error like so:
AFW caused an invalid page fault in
module AFW.EXE at 014f:00413750.
Registers:
EAX=0042fdff CS=014f EIP=00413750 EFLGS=00010286
EBX=00560000 SS=0157 ESP=0066fb68 EBP=0066fb74
ECX=cccccccc DS=0157 ESI=cccccccc FS=3faf
EDX=0066fc48 ES=0157 EDI=0066fc48 GS=0000
Bytes at CS:EIP:
8a 06 46 8a 27 47 38 c4 74 f2 2c 41 3c 1a 1a c9
Stack dump:
00560000 81668408 0066fbe4 0066fbe4 004011e3 0066fc48 cccccccc 0066fcb8 81668408 00560000 cccccccc cccccccc cccccccc cccccccc cccccccc cccccccc


I think this means I have a memory leak that's being caused by one of the two loops. I have no idea why this is happening... the functions and data types used here work flawlessly outside of the Import function, so I don't think that they are to blame.

If anyone can make heads or tails of this, I'd really appreciate it.

Steve

You will have to post more code...the only thing I see wrong with the code that you posted is the "while (infile)" loop. This is been a popular topic of late - check out this post.

gg

What extra code should I post?

Well, if you want someone to find the problem for you - all of it (zip it up and attach).
Or you can execute the program in a debugger and track down the problem (which is what I will do if you post the code). If you're not familiar will using a debugger to track down such issues, this is a good problem to learn on.

gg

Heres the zip, with all the cpp and header files (and the rest of the stuff that MSVC++6 creates when it compiles).

BTW, any good tutorials anywhere on how to use the debugger? It confuses me, to put it lightly.

I'll post something on how to use VC++ debugger after I look at your code.

gg

Ok, I found it. The problem is that your are assuming that the compiler will initialize your data for you. For example:

Code:

---

int n; //can't assume the value will be 0, or any other value for that matter

---

The same thing goes for struct's and class's:

Code:

---

struct foo
{
  int n;
  int *p;
};
...
foo f; //can't assume any value for f.n or f.p

---

So, you are getting a runtime error because FromList->head does not equal NULL when Add(FromList, ImportedTitle) is run. This easily fixed by adding constructors to your structs:

Code:

---

struct Movie
{
    String Title;
    String Renter;

    Movie* Next;

    Movie() : Next(NULL) {}
};

struct MovieList
{
    Movie* Head;
    int Size;
   
    MovieList() : Head(NULL),Size(0) {}
};

---

As for using the debugger, its easier than you think. The debugger will basically allow you to inspect the values of your variables and stop/start execution. Put your cursor on "Add(FromList, ImportedTitle);" then select Build->Start Debug->Run To Cursor.
Type in 7 and movies.txt and you'll see that execution has stopped right where you told it. You can now inspect the values of your variables by selecting View->Debug Windows->Variables.
Click on the '+' next to 'FromList'. You will see that FromList->Head has a value of 0xcccccccc and not NULL.
The best way to learn the debugger is to play around with it.

gg

You're a lifesaver! Many, many thanks.