test.c

#include <stdlib.h>
#include <stdio.h>

typedef struct {
    int structInt;
    char *structString;
} myStructType;

void changeStruct(myStructType *aStructPointer) {
    myStructType *aNewStructPointer = (myStructType *)malloc(sizeof(myStructType));

    aNewStructPointer->structInt = 1;
    aNewStructPointer->structString = "New String";

    aStructPointer = aNewStructPointer;
}

int main(int argc, char *argv[]) {
    myStructType *aStructPointer = (myStructType *)malloc(sizeof(myStructType));

    aStructPointer->structInt = 0;
    aStructPointer->structString = "Old String";

    printf("Before swap: {%i, \"%s\"}\n", aStructPointer->structInt, aStructPointer->structString);
    changeStruct(aStructPointer);
    printf("After swap: {%i, \"%s\"}\n", aStructPointer->structInt, aStructPointer->structString);

    return 0;
}

test.m

#import <Cocoa/Cocoa.h>

void swapString(NSString *aString) {
    NSString *aNewString = [NSString stringWithFormat:@"New String"];
    aString = aNewString;
}

int main(int argc, char *argv[]) {
    NSAutoreleasePool *localPool = [[NSAutoreleasePool alloc] init];
    NSString *oldString = @"Old String";

    NSLog(@"%@", oldString);
    swapString(oldString);
    NSLog(@"%@", oldString);
    
    [localPool drain];
    return 0;
}