ES101 - Lesson 8 : Functions with Pass-By-Reference

Objective

You will learn the basics of pointers, which are used in combination with the functions.

Pass-by-value type of functions pass just the value of the variables into a function, but with a simple change, you can use functions that accept pass-by-reference type of variables. This is a very powerful concept that will allow your functions to change the values outside of its scope.

Pass By Reference

Pass by reference provides a mean to provide input to functions by specifying a memory location, which allows the function to modify caller's variables. In other words, instead of just merely giving a value, you pass the pointer of variable(s), and the pointer points to the memory address where the data is stored. The values are then accessed by using the * operator inside the function, which is called the dereference operator. Students often get confused with & and * symbols. & refers to the memory address whereas as * refers to a value stored at this memory address.

// Declare foobar() that takes pointers of int and float
void foobar(int *intInput, float *fpInput);

int main(void)
{
   // We can no longer do this since we have to provide
   // addresses of some variable
   foobar(1, 1.2);        // ERROR!
   foobar(anInt, aFloat); // ERROR!

   // Pass variable addresses to foobar()
   int anInt = 1;
   float aFloat = 2.3;
   foobar(&anInt, &aFloat);

   // After function call to foobar(): anInt and aFloat are zero
   printf("Variables now: %i, %f\n", anInt, aFloat);
}

void foobar(int *intInput, float *fpInput)
{
   // intInput and fpInput are addresses, so to read/write
   // the values at these addresses, we must use * (dereference)
   // operator
   printf("Input was: %i, %f\n", *intInput, *fpInput);

   // Modify the inputs:
   *intInput = 0;
   *fpInput  = 0;
}