Objc中2维指针作为输出参数时由ARC及@autoreleasepool引发的血案

先看下面一个例子

#import <UIKit/UIKit.h>

#import "AppDelegate.h"

@interface Something : NSObject

- (void)doWithError:(NSError **)outError;

@end

@implementation Something

- (void)doWithError:(NSError **)outError
{
    @autoreleasepool
    {
        *outError = [NSError errorWithDomain:@"Emergency" code:999 userInfo:nil];
    }
}

@end

int main(int argc, const char *argv[])
{
    @autoreleasepool
    {
        NSError *error = nil;

　　　　　// Sometimes EXC_BAD_ACCESS when return from this method.
        [[[Something alloc] init] doWithError:&error];

        // At this point, the main thread gives EXC_BAD_ACCESS.
        NSLog(@"%@", error);
    }
    return 0;
}

在NSLog输出的时候会EXC_Bad_ACCESS

正确写法应该是这样：

@interface Something : NSObject

- (void)doWithError:(NSError **)outError;

@end

@implementation Something

- (void)doWithError:(NSError **)outError
{
    NSError *error = nil;
    @autoreleasepool
    {
        error = [NSError errorWithDomain:@"Emergency" code:999 userInfo:nil];
        // Do something
    }
    if(error)
    {
        *outError = error;
    }
}

@end

int main(int argc, char * argv[])
{
    @autoreleasepool
    {
        NSError *__autoreleasing error = nil;

        [[[Something alloc] init] doWithError:&error];

        // At this point, the main thread gives EXC_BAD_ACCESS.
        NSLog(@"%@", error);

        int a = 1;
        ++a;
    }
    return 0;
}