Objective C Pointers - objective-c

Question

I am experiencing a very odd problem with pointers. As you could see from the below code, I am using a method that generates a random 4 by 4 character grid. It return a pointer to a two dimensional character array. The problem is that when i try to assign the returned pointer to another pointer and try to print the generated grid, I get just one strange symbol.
Header File
#import <Foundation/Foundation.h>
#interface GridGenerator : NSObject
{
}
-(char (*)[4]) generateGrid;
-(int (*)[2]) bbb;
-(void) print;
#end
Implementation File
#import "GridGenerator.h"
#implementation GridGenerator
-(char (*)[4])generateGrid{
char vowels[6] = {'A','E','I','O','U','Y'};
char consonants[20] = {'B','C','D','F','G','H','J','K','L','M','N','P','Q','R','S','T','V','W','X','Z'};
char grid[4][4];
int vowelsLength = (sizeof vowels / sizeof vowels[0]);
int consLength = (sizeof consonants / sizeof consonants[0]);
int gridSize = (sizeof grid / sizeof grid[0]);
for(int i=0;i<gridSize;i++){
int vowelsInGridRow = 0;
int noOfVowels = (arc4random() % 2) + 1;
for(int j=0;j<gridSize;j++){
if(noOfVowels != vowelsInGridRow){
int vowIndex = arc4random() % vowelsLength;
char s = vowels[vowIndex];
grid[i][j] = s;
vowelsInGridRow++;
}
else{
int consIndex = arc4random() % consLength;
char s = consonants[consIndex];
grid[i][j] = s;
}
}
}
char (*sd)[4]= grid;
return sd;
}
-(void)print{
char (*grid)[4] = [self generateGrid];
NSString *s = #"\n";
for(int i=0;i<4;i++){
for(int j=0;j<4;j++){
s = [s stringByAppendingString:[NSString stringWithFormat:#"%c",grid[i][j]]];
}
s = [s stringByAppendingString:#"\n"];
}
NSLog(#"%#",s);
}
Main File(Test)
#import <Foundation/Foundation.h>
#import "Crossword.h"
#import "GridGenerator.h"
int main(int argc, const char * argv[])
{
NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
GridGenerator *gen = [[GridGenerator alloc] init];
[gen print];
[pool release];
return 0;
}
When I run the code, you can see the result below. After the 'U' there is an inverted question mark(could not be pasted here).
2013-06-02 11:24:29.923 CrosswordTest[646:303]
U
Do you have an idea, what could causes this to happen? I am struggling already for several hours and I cannot find any explanation.

Answer 1

You are returning a reference to a temporary. Explosion (undefined behavior) should be expected.
Workaround. Create a structure:
typedef struct {char at[4][4];} t_grid;
Then populate and return the t_grid by value:
- (t_grid)generateGrid
{
char vowels[6] = {'A','E','I','O','U','Y'};
char consonants[20] = {'B','C','D','F','G','H','J','K','L','M','N','P','Q','R','S','T','V','W','X','Z'};
t_grid grid;
...
int gridSize = (sizeof grid.at / sizeof grid.at[0]);
...
grid.at[i][j] = s;
...
return grid;
}
- (void)print
{
t_grid grid = [self generateGrid];
...
Note that you should not use this approach for large arrays or variable length arrays. 4*4 octets is small.