reading a file of unknown length with a function

Question

I'm trying to write a short function that will let me quickly read in a file of unknown size and return pointer to the array of data and the length of that array but it seems my code isn't working. What am i doing wrong?

int readIn(int* pointer, param parameters, string description)
{
    string fileName = parameters.fileName + " " + description + ".bin";

    ifstream readFile;
    readFile.open(fileName.c_str(), ios::in|ios::binary|ios::ate);  

    int size = readFile.tellg();
    int length = size / 4;
    int* output = new int [length];

    readFile.seekg (0, ios::beg);
    readFile.read(reinterpret_cast<char*>(output), (size));
    readFile.close();

    pointer = output; // link new array with the pointer
    return length;
}

and in the main function:

int* testList;
int numEntries = readIn(testList, parameters, "test");

I end up with an error saying that my testList variable was used and not initialized. What am i doing wrong?

Answer 1

You aren't returning anything in your pointer variable after the function call.

You can fill a variable so it's value remains changed after the function call by dereferencing it's address.

Example:

void fillX(int *p)
{
  //p holds a memory address, go to that memory address and change its value
  *p = 4;
}


void main(int argc, char **argv)
{
  int x;
  fillX(&x);
  return 0;
  assert(x == 4);
}

To change what a pointer points to you need to pass in a pointer to a pointer.

I.e. you'd need to pass in the address of your pointer, and then you'd need the parameter type to be an int** pointer. When you set it you'd say *pointer = buffer;

Example:

void fillPointer(int **pp)
{
  //p holds a memory address to a pointer 
  //Go to that memory address and change its value
  *pp = new int[10];
}


void main(int argc, char **argv)
{
  int *x;
  fillPointer(&x);
  //x now points to the first element of an array
  delete[] x;
  return 0;
}

The key point here: When you want to change values via a parameter, you need to pass in it's address, then dereference it to set what's at that address.

Answer 2

int readIn(int** pointer, param parameters, string description)

...

*pointer = output

 ...

int numEntries = readIn(&testList, ...)

Answer 3

I could be wrong here but perhaps you need to make your function,

int readIn(int **pointer, param parameters, string description)

, i.e. a pointer to a pointer. Call it with,

int numEntries = readIn(&testList, parameters, "test");

vars are passed by value in C/C++.