Question

I’m reading a book to learn Objective-C and this program is suppose to show key concepts in dealing with pointers, and I’m really lost.

Is there some kind of conversion happening in the function’s arguments that turn p1, p2, &il, and &i2 to the value (*) of a pointer? Like p1 turns into *p1?

I thought a copy of the variable was passed into the function instead of the actual variable, so why was the value of the passed in variable changed after the function?

Also why am I getting a warning on the 3rd line that says: No previous prototype for function 'exchangeValues'?

Thank you!!

#import <Foundation/Foundation.h>

void exchangeValues (int *pint1, int *pint2) {
    int temp;

    temp = *pint1;
    *pint1 = *pint2;
    *pint2 = temp;
}

int main (int argc, char *argv[]) {
    @autoreleasepool {
        void exchangeValues (int *pint1, int *pint2);
        int il = -5, i2 = 66, *p1 = &il, *p2 = &i2;

        NSLog(@"il = %i, i2 = %i", il, i2);

        exchangeValues(p1, p2);
        NSLog(@"il = %i, i2 = %i", il, i2);

        exchangeValues(&il, &i2);
        NSLog(@"il = %i, i2 = %i", il, i2);
    }
    return 0;
}

---

Output:

2012-08-02 11:13:38.569 Test[381:707] il = -5, i2 = 66
2012-08-02 11:13:38.571 Test[381:707] il = 66, i2 = -5
2012-08-02 11:13:38.572 Test[381:707] il = -5, i2 = 66

Answer 1

I would say that’s a complex example if you are being taught about pointers!

Is there some kind of conversion happening in the function’s arguments
that turn p1, p2, &il, and &i2 to the value (*) of a pointer? Like p1
turns into *p1?

p1 and p2 are declared as int * (pointer to int) and are initialised with the address of i1 and i2 (using the & operator).

I thought a copy of the variable was passed into the function instead
of the actual variable, so why was the value of the passed in variable
changed after the function?

A copy of the variable is passed to the function, however in this case the variable of type int * (pointer to int). The reason the value is changing is because the exchangeValues() function is dereferencing those pointers and swapping the values. This is the only way (in C/Objective-C) a function can modify a variable outside of its own scope, other than the variable being assigned as the return value from a function.

Also why am I getting a warning on the 3rd line that says: No previous
prototype for function ‘exchangeValues’?

You seem to have typed it in wrong; remove the line below @autoreleasepool:

@autoreleasepool {
    void exchangeValues (int *pint1, int *pint2);   <-- delete this line