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Soulstorm · Apr 27th, 2006, 8:31 AM

I have post it into another forum as well, but I would like some answers here, too.

I am currently a newbie to Obj-C, and I have a strong background in C++. Here are some lines of code that I would like to have explained:

(Toggle Plain Text)

int main(int argc, char* argv[]){
	fraction *a = [[fraction alloc] init];
	fraction *b = [[fraction alloc] init];

	fraction *result;
	
	[a setTo: 1 over: 3];
	[b setTo: 2 over: 5];
	[a print];
	printf(" + ");
	[b print];
	printf(" = ");
	result = [a add: b];
	[result print];
	printf("\n");
	[result free];

	[a print];
	printf(" - ");
	[b print];
	printf(" = ");
	result = [a sub: b];
	[result print];
	printf("\n");
	[result free];
	
	[a print];
	printf(" * ");
	[b print];
	printf(" = ");
	result = [a mul: b];
	[result print];
	printf("\n");
	[result free];
	
	[a print];
	printf(" / ");
	[b print];
	printf(" = ");
	result = [a div: b];
	[result print];
	printf("\n");
	[result free];
	
	return 0;
}

You will probably understand that I have a fraction class declared elsewhere and some methods, which are of no importance to my question.

As you can see, I am pre-allocating the "fraction *a" and "fraction *b" but NOT "fraction *result". Why is that? How come it's not necessary to write

(Toggle Plain Text)

fraction *result = [[fraction alloc]init]

to allocate memory for the "result" object?

I know that if I eventually write "fraction *result = [[fraction alloc]init]" it will also be correct, but why doesn't it give me any errors?

NOTE: In the "-(void) setTo: (int) n over: (int) d" function that you see into the "main()" I do not allocate memory for my object

In general, I have a hard time trying to figure out when must I allocate objects using the "alloc" method and when not. I also have a hard time to avoid memory leaks (for example, I know that I shouldn't write "[[a div: b] print]" because an object will be created that I won't be able to free later).

If you still can't figure out what I am saying, here is the entire source.

sykkn · Apr 27th, 2006, 10:39 AM

(Toggle Plain Text)

-(fraction *) add: (fraction *) f
{
	fraction *result = [[fraction alloc] init];
	int resultNum, resultDenom;
	
	resultNum = (numerator * [f denominator])
				+(denominator * [f numerator]);
	resultDenom = denominator * [f denominator];
	[result setTo:resultNum over:resultDenom];
	[result reduce];
	return result;
}

there is your answer ... you do not need to allocate for result because it is allocated in a's add method ... then a pointer to the result is passed back and stored in your result var.

Soulstorm · Apr 27th, 2006, 12:57 PM

The result that I am talking about isn't initialized and allocated. The 'result' in main() itself is not allocated nor initialized in the 'add' method. Inside the method, a new object is created, and then it is initialized, but the fact that they have the same name with the 'result' in the main() doesn't mean they are the same object.

...or am I wrong?

Also, someone said I have produced a memory leak here... Could you explain me why? the other thread is here

Kaja Fumei · Apr 27th, 2006, 2:44 PM

Yes, the result you are talking about is initialized and allocated several times, just not in main. When main runs this line:

(Toggle Plain Text)

result = [a add: b];

it calls the add method found in the "fraction.m" file:

(Toggle Plain Text)

-(fraction *) add: (fraction *) f
{
	fraction *result = [[fraction alloc] init];  // point of interest #1
	
        // skipping over the rest of the computation because its not necessary
        // to the explanation

	return result;    // point of interest #2
}

The very first line creates a local variable called "result" which is a pointer to a fraction (the fact that it has the same name as the variable in main is irrelevant), and allocates a new fraction and stores the pointer to the new fraction in that local variable. The add method does its computations, and at the end, returns the pointer stored in the local "result". Then, according to the first line, I posted, that pointer is stored in main's "result" variable.
The after main's result is printed, main calls:

(Toggle Plain Text)

[result free];

which frees the allocated memory stored pointed to by result. Then as you perform the subtraction, multiplication, and division, the same cycle goes on: the methods return the result with new allocated memory which is freed after its printed.

On a side note, your code never frees "a" and "b" so add:

(Toggle Plain Text)

[a free];
[b free];

above the return 0 in main.

Soulstorm · Apr 28th, 2006, 4:57 AM

Thanks a lot, that was quite helpful.

Quote:

On a side note, your code never frees "a" and "b" so add:

(Toggle Plain Text)

[a free];
[b free];

above the return 0 in main.

Heh. I figured that out later. Thanks a lot.

Apr 27th, 2006, 12:57 PM	#3
Soulstorm Hobbyist Programmer Join Date: Jan 2006 Location: Menidi, Athens, Greece Posts: 243 Rep Power: 3	The result that I am talking about isn't initialized and allocated. The 'result' in main() itself is not allocated nor initialized in the 'add' method. Inside the method, a new object is created, and then it is initialized, but the fact that they have the same name with the 'result' in the main() doesn't mean they are the same object. ...or am I wrong? Also, someone said I have produced a memory leak here... Could you explain me why? the other thread is here __________________ Project::Soulstorm (personal homepage)

Apr 27th, 2006, 2:44 PM	#4
Kaja Fumei Hobbyist Programmer Join Date: Oct 2005 Posts: 134 Rep Power: 4	Yes, the result you are talking about is initialized and allocated several times, just not in main. When main runs this line: (Toggle Plain Text) result = [a add: b]; result = [a add: b]; it calls the add method found in the "fraction.m" file: (Toggle Plain Text) -(fraction ) add: (fraction ) f { fraction result = [[fraction alloc] init]; // point of interest #1 // skipping over the rest of the computation because its not necessary // to the explanation return result; // point of interest #2 } -(fraction ) add: (fraction ) f { fraction result = [[fraction alloc] init]; // point of interest #1 // skipping over the rest of the computation because its not necessary // to the explanation return result; // point of interest #2 } The very first line creates a local variable called "result" which is a pointer to a fraction (the fact that it has the same name as the variable in main is irrelevant), and allocates a new fraction and stores the pointer to the new fraction in that local variable. The add method does its computations, and at the end, returns the pointer stored in the local "result". Then, according to the first line, I posted, that pointer is stored in main's "result" variable. The after main's result is printed, main calls: (Toggle Plain Text) [result free]; [result free]; which frees the allocated memory stored pointed to by result. Then as you perform the subtraction, multiplication, and division, the same cycle goes on: the methods return the result with new allocated memory which is freed after its printed. On a side note, your code never frees "a" and "b" so add: (Toggle Plain Text) [a free]; [b free]; [a free]; [b free]; above the return 0 in main.

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Apr 27th, 2006, 8:31 AM	#1
Soulstorm Hobbyist Programmer Join Date: Jan 2006 Location: Menidi, Athens, Greece Posts: 243 Rep Power: 3	Objective-C: When do I need to Allocate? I have post it into another forum as well, but I would like some answers here, too. I am currently a newbie to Obj-C, and I have a strong background in C++. Here are some lines of code that I would like to have explained: (Toggle Plain Text) int main(int argc, char* argv[]){ fraction a = [[fraction alloc] init]; fraction b = [[fraction alloc] init]; fraction result; [a setTo: 1 over: 3]; [b setTo: 2 over: 5]; [a print]; printf(" + "); [b print]; printf(" = "); result = [a add: b]; [result print]; printf("\n"); [result free]; [a print]; printf(" - "); [b print]; printf(" = "); result = [a sub: b]; [result print]; printf("\n"); [result free]; [a print]; printf(" "); [b print]; printf(" = "); result = [a mul: b]; [result print]; printf("\n"); [result free]; [a print]; printf(" / "); [b print]; printf(" = "); result = [a div: b]; [result print]; printf("\n"); [result free]; return 0; } int main(int argc, char* argv[]){ fraction a = [[fraction alloc] init]; fraction b = [[fraction alloc] init]; fraction result; [a setTo: 1 over: 3]; [b setTo: 2 over: 5]; [a print]; printf(" + "); [b print]; printf(" = "); result = [a add: b]; [result print]; printf("\n"); [result free]; [a print]; printf(" - "); [b print]; printf(" = "); result = [a sub: b]; [result print]; printf("\n"); [result free]; [a print]; printf(" "); [b print]; printf(" = "); result = [a mul: b]; [result print]; printf("\n"); [result free]; [a print]; printf(" / "); [b print]; printf(" = "); result = [a div: b]; [result print]; printf("\n"); [result free]; return 0; } You will probably understand that I have a fraction class declared elsewhere and some methods, which are of no importance to my question. As you can see, I am pre-allocating the "fraction a" and "fraction b" but NOT "fraction result". Why is that? How come it's not necessary to write (Toggle Plain Text) fraction result = [[fraction alloc]init] fraction result = [[fraction alloc]init] to allocate memory for the "result" object? I know that if I eventually write "fraction result = [[fraction alloc]init]" it will also be correct, but why doesn't it give me any errors? NOTE: In the "-(void) setTo: (int) n over: (int) d" function that you see into the "main()" I do not allocate memory for my object In general, I have a hard time trying to figure out when must I allocate objects using the "alloc" method and when not. I also have a hard time to avoid memory leaks (for example, I know that I shouldn't write "[[a div: b] print]" because an object will be created that I won't be able to free later). If you still can't figure out what I am saying, here is the entire source. __________________ Project::Soulstorm (personal homepage)

Apr 27th, 2006, 10:39 AM	#2
sykkn Hobbyist Programmer Join Date: Apr 2004 Location: Texas Posts: 106 Rep Power: 5	(Toggle Plain Text) -(fraction ) add: (fraction ) f { fraction result = [[fraction alloc] init]; int resultNum, resultDenom; resultNum = (numerator [f denominator]) +(denominator * [f numerator]); resultDenom = denominator * [f denominator]; [result setTo:resultNum over:resultDenom]; [result reduce]; return result; } -(fraction ) add: (fraction ) f { fraction result = [[fraction alloc] init]; int resultNum, resultDenom; resultNum = (numerator [f denominator]) +(denominator * [f numerator]); resultDenom = denominator * [f denominator]; [result setTo:resultNum over:resultDenom]; [result reduce]; return result; } there is your answer ... you do not need to allocate for result because it is allocated in a's add method ... then a pointer to the result is passed back and stored in your result var. __________________ [ [ SykkN alloc ] initWithThePowerTo: destroyYouAll ]; /* Don't make me use it! */

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Objective-C: When do I need to Allocate?