Objective-C dealloc 方法深度解析與釋放順序實戰

dealloc 方法核心職責

當物件參考計數歸零，系統自動呼叫 dealloc 釋放記憶體。標準實作模式：

- (void)dealloc {
    // 1. 釋放自有實例變數
    [_cpu release];
    // 2. 呼叫父類別 dealloc
    [super dealloc];
}

為何必須呼叫 [super dealloc]？

子類別繼承父類別的實例變數，必須由父類別負責釋放，確保完整清理繼承鏈上的所有資源。

釋放順序原則：先子後父

與初始化順序（先父後子）相反，釋放時先清理子類別資源，再呼叫 [super dealloc] 讓父類別釋放其資源。

完整範例：Vehicle → Car 繼承關係

// Vehicle.h
#import <Foundation/Foundation.h>
@interface Vehicle : NSObject {
    NSString *_name;
}
- (id)initWithName:(NSString *)name;
@end

// Vehicle.m
#import "Vehicle.h"
@implementation Vehicle
- (id)initWithName:(NSString *)name {
    self = [super init];
    if (self) _name = [name copy];
    return self;
}
- (void)dealloc {
    NSLog(@"vehicle dead");
    [_name release];
    [super dealloc];
}
@end

// Car.h
#import <Foundation/Foundation.h>
#import "Vehicle.h"
@class Engine;
@interface Car : Vehicle {
    Engine *_engine;
}
- (void)setEngine:(Engine *)engine;
@end

// Car.m
#import "Car.h"
#import "Engine.h"
@implementation Car
- (void)setEngine:(Engine *)engine {
    if (_engine != engine) {
        [_engine release];
        _engine = [engine retain];
    }
}
- (void)dealloc {
    NSLog(@"Car dead");
    [_engine release];
    [super dealloc];
}
@end

// main.m
#import <Foundation/Foundation.h>
#import "Engine.h"
#import "Car.h"
int main(int argc, const char * argv[]) {
    @autoreleasepool {
        NSString *name = [[NSString alloc] initWithFormat:@"audi"];
        Car *car = [[Car alloc] initWithName:name];
        [name release];
        Engine *v6 = [[Engine alloc] init];
        [car setEngine:v6];
        [v6 release];
        [car release]; // 觸發 dealloc 鏈
    }
    return 0;
}

輸出順序為 Car dead → vehicle dead，驗證釋放順序正確。