第九块石头

ARC中精确的对象生存期

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从WWDC 2012 Session 712 Asynchronous design patterns with blocks gcd and xpc 里看到的，用其它形式没有重现，暂且记下来。

代码

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- (void)logWithData:(dispatch_data_t)data {
  void *buf;
  dispatch_data_t map;

  map = dispatch_data_create_map(data, &buf, NULL);
  NSLog(@"%@", [NSString stringWithUTF8String:buf]);
}

将会被编译为

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- (void)logWithData:(dispatch_data_t)data {
  void *buf;
  dispatch_data_t map;

  map = dispatch_data_create_map(data, &buf, NULL);
  objc_release(map);
  NSLog(@"%@", [NSString stringWithUTF8String:buf]);
}

这种情况往往会造成崩溃，需要对对象的生存时间进行严格定义。

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- (void)logWithData:(dispatch_data_t)data {
  void *buf;
  dispatch_data_t map __attribute__((objc_precise_lifetime));

  map = dispatch_data_create_map(data, &buf, NULL);
  NSLog(@"%@", [NSString stringWithUTF8String:buf]);
}

上面代码将会编译成

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- (void)logWithData:(dispatch_data_t)data {
  void *buf;
  dispatch_data_t map __attribute__((objc_precise_lifetime));

  map = dispatch_data_create_map(data, &buf, NULL);
  NSLog(@"%@", [NSString stringWithUTF8String:buf]);
  objc_releas(map);
}

刚才我试了下面一段代码，发现没有崩溃，不知道里面哪里多了autorelease

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char *buffer = malloc(30);
NSData *data = [[NSData alloc] initWithBytesNoCopy:buffer length:30 freeWhenDone:YES];

buffer[0] = 'a';
buffer[1] = 0;
NSLog(@"%s", buffer);

理论上应该是编译成这样：

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char *buffer = malloc(30);
NSData *data = [[NSData alloc] initWithBytesNoCopy:buffer length:30 freeWhenDone:YES];
objc_releas(data);
buffer[0] = 'a';
buffer[1] = 0;
NSLog(@"%s", buffer);

很奇怪。

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