ARC下NSException可能会内存泄露

在使用Instrumnents对程序进行内存测试的时候，发现有几处异常处理的地方提示内存泄露。经过分析得知，因为异常处理，改变代码执行路径，导致编译生成的 release 代码没有执行。

简单写了些测试代码验证：

- (void)viewDidLoad {
    
    [super viewDidLoad];
    
    @try {
        NSMutableArray *stringList = [[NSMutableArray alloc] init];
        for (int i = 0; i < 100; i++) {
            [stringList addObject: @"string instance."];
        }
        [self throwException];
    }
    @catch (NSException *exception) {
        NSLog(@"catch Exception : %@", exception);
    }
    @finally {
        
    }
}

- (void)throwException
{
    [NSException raise:@"An Exception." format:@"Exception Description."];
}

反复进出这个页面几次后，在Instruments上果然看到提示内存泄露，而且泄露的地方就指明是 @try{ } 里面创建的对象。

memory-leak

解决思路

1, 尽量不要在@try{}里面有操作，导致对象的引用引用计数增加。@try{ } 语句块里面只有有可能抛出异常的语句。

修改后的代码:

- (void)viewDidLoad {
    
    [super viewDidLoad];
    
    NSMutableArray *stringList = [[NSMutableArray alloc] init];
    for (int i = 0; i < 100; i++) {
        [stringList addObject: @"string instance."];
    }
    
    @try {
        [self throwException];
    }
    @catch (NSException *exception) {
        NSLog(@"catch Exception : %@", exception);
    }
    @finally {
        
    }
}

最后用Instruments检查一下，一切正常了。

2, 查看了苹果的开发文档，发现有这么一段描述:

By default in Objective C, ARC is not exception-safe for normal releases:
It does not end the lifetime of __strong variables when their scopes are abnormally terminated by an exception.
It does not perform releases which would occur at the end of a full-expression if that full-expression throws an exception.
A program may be compiled with the option -fobjc-arc-exceptions in order to enable these, or with the option -fno-objc-arc-exceptions to explicitly disable them, with the last such argument “winning”.