Question

I have a trouble reading a .txt file into a char* array.

I have a levels.txt file that looks like this:

level1.txt
level2.txt

I have my array defined inside a class as

 char* levels[10];

And my parsing function looks like this:

// Parse the level list file
int Environment::parseLevels() {
    ifstream data;
    data.open("levels.txt");

    char buf[64];

    for (int i=0; i<sizeof(levels); i++) {
        data.getline(buf, 64);
        levels[0] = strtok(buf, " ");
    }
}

If I do cout << levels[0]; right after

levels[0] = strtok(buf, " ");

then I get a fine output. However, when I try to cout << levels[0]; from somewhere else, nothing gets displayed.

What am I doing wrong?

Thank you in advance!

Answer 1

The pointer returned by strtok is not going to point to valid memory forever since the buffer that you’re tokenizing is declared on the stack. You’ll need to actually copy the string being pointed to by the return pointer of strtok, not the pointer itself if you want to use the string outside the function body.

So basically modify your code to the following:

levels[0] = new char[64];
char* temp = strtok(buf, " ");

//check for NULL pointer return from strtok()
if (temp) 
{
    //if the pointer is not NULL, copy the contents of the temporary string
    //returned by strtok into more permanent memory allocated on the heap
    //and being pointed to by levels[0]

    //Use strncpy() to prevent the risk of a buffer overflow
    strncpy(levels[0], temp, 64);
}

Then in the destructor for your Environment object, make sure to have some loop to free the memory being pointed to by each of the members of the levels array that are pointing to memory allocated via new []. You do this by calling delete []