[Objective C] 11 Retain and Release

Retain and Release
Retain and release are two methods inherited from any object that has NSObject as a parent. Each object has an internal counter that can be used to keep track of the number references an object has. So if you have 3 referneces, you don't want to dealloc yourself. However once you reach 0, you should dealloc yourself. [object retain] increments the counter by 1 (which starts at 1) and [object release] decrements it by 1. If the [object release] invocation causes the count to reach 0, dealloc is then called.

alloc 하면 1, retain 하면 +1, release -1, 0 되면 자동 dealloc 콜.
주소만 참조하는 걸로는 +1 안됨.

Fraction.h

#import <Foundation/NSObject.h> @interface Fraction: NSObject {     int numerator;     int denominator; } -(Fraction*) initWithNumerator: (int) n andDenominator: (int) d; -(Fraction*) initWithNumerator: (int) n denominator: (int) d; -(void) print; -(void) setNumerator: (int) n; -(void) setNumerator: (int) n andDenominator: (int) d; -(void) setDenominator: (int) d; -(int) numerator; -(int) denominator; @end

Fraction.m

#import "Fraction.h" #import <stdio.h> @implementation Fraction -(Fraction*) initWithNumerator: (int) n andDenominator: (int) d {     self = [super init];         if(self) {         [self setNumerator: n andDenominator: d];     }         return self; } -(Fraction*) initWithNumerator: (int) n denominator: (int) d {     self = [super init];         if(self) {         [self setNumerator: n andDenominator: d];     }         return self; } -(void) print {     printf( "%i/%i", numerator, denominator ); } -(void) setNumerator: (int) n {     numerator = n; } -(void) setNumerator: (int) n andDenominator: (int) d {     numerator = n;     denominator = d; } -(void) setDenominator: (int) d {     denominator = d; } -(int) denominator {     return denominator; } -(int) numerator {     return numerator; } // dealloc -(void) dealloc {     printf( "Deallocing fraction\n" );     [super dealloc]; } @end

main.m

#import "Fraction.h" #import <stdio.h> int main( int argc, const char *argv[] ) {     Fraction *frac1 = [[Fraction alloc] init];     Fraction *frac2 = [[Fraction alloc] init];     // print current counts     printf( "Fraction 1 retain count: %i\n", [frac1 retainCount] );     printf( "Fraction 2 retain count: %i\n", [frac2 retainCount] );     // increment them     [frac1 retain]; // 2     [frac1 retain]; // 3     [frac2 retain]; // 2     // print current counts     printf( "Fraction 1 retain count: %i\n", [frac1 retainCount] );     printf( "Fraction 2 retain count: %i\n", [frac2 retainCount] );     // decrement     [frac1 release]; // 2     [frac2 release]; // 1     // print current counts     printf( "Fraction 1 retain count: %i\n", [frac1 retainCount] );     printf( "Fraction 2 retain count: %i\n", [frac2 retainCount] );         // release them until they dealloc themselves     [frac1 release]; // 1     [frac1 release]; // 0     [frac2 release]; // 0         // 다른 변수가 참조하면?     Fraction *frac3 = [[Fraction alloc] init];     printf( "Fraction 3 retain count: %i\n", [frac3 retainCount] );     [frac3 retain]; // 2     printf( "Fraction 3 retain count: %i\n", [frac3 retainCount] );     Fraction* frac4 = frac3; // 참조해도 retainCount가 늘어나지는 않음.     printf( "Fraction 3 retain count: %i\n", [frac3 retainCount] );     [frac3 release]; // 1     printf( "Fraction 3 retain count: %i\n", [frac3 retainCount] ); // 1     printf( "Fraction 4 retain count: %i\n", [frac4 retainCount] ); // 1     system("PAUSE");     [frac4 release]; // 0 - frac3 또는 frac4 둘 중 하나만 release 해도 retainCount가 감소. 같은 주소이므로 당연.     system("PAUSE"); // retainCount가 0이 되었으므로 dealloc 됨.     printf( "Fraction 3 retain count: %i\n", [frac3 retainCount] ); // dealloc된 frac3을 참조하려하므로 런타임 예외 발생 //    system("PAUSE"); //    printf( "Fraction 4 retain count: %i\n", [frac4 retainCount] );     system("PAUSE");     [frac3 release]; // 0         system("PAUSE");     return 0; }

결과

Fraction 1 retain count: 1 Fraction 2 retain count: 1 Fraction 1 retain count: 3 Fraction 2 retain count: 2 Fraction 1 retain count: 2 Fraction 2 retain count: 1 Deallocing fraction Deallocing fraction Fraction 3 retain count: 1 Fraction 3 retain count: 2 Fraction 3 retain count: 2 Fraction 3 retain count: 1 Fraction 4 retain count: 1 계속하려면 아무 키나 누르십시오 . . . Deallocing fraction 계속하려면 아무 키나 누르십시오 . . .

The retain calls increment the counter. The release calls decrement it. One can get the count as an int by calling [obj retainCount]. Once the retainCount reaches 0, both objects dealloc themselves and you can see this when both print out "Deallocing fraction."