是（变量）与Objective-C中的if（变量！= nil）相同

Question

I am getting a EXC_BAD_ACCESS (SIGBUS) on this line in my iPhone project:

if (timeoutTimer) [timeoutTimer invalidate];

The thing that has me stumped is that I don't understand how that line could crash, since the if statement is meant to be checking for nil. Am I misunderstanding the way Objective-C works, or do line numbers in crash statements sometime have the wrong line in them?

Answer 1

Just because a variable is set to a value other than nil doesn't mean it's pointing to a valid object. For example:

id object = [[NSObject alloc] init];
[object release];
NSLog(@"%@", object); // Not nil, but a deallocated object,
                      // meaning a likely crash

Your timer has probably already been gotten rid of (or possibly hasn't been created at all?) but the variable wasn't set to nil.

Answer 2

I just ran into a similar issue, so here's another example of what might cause a check such as yours to fail.

In my case, I was getting the value from a dictionary like this:

NSString *text = [dict objectForKey:@"text"];

Later on, I was using the variable like this:

if (text) {
    // do something with "text"
}

This resulted in a EXC_BAD_ACCESS error and program crash.

The problem was that my dictionary used NSNull values in cases where an object had an empty value (it had been deserialized from JSON), since NSDictionary cannot hold nil values. I ended up working around it like this:

NSString *text = [dict objectForKey:@"text"];
if ([[NSNull null] isEqual:text]) {
    text = nil;
}

Answer 3

They should be the same. Perhaps the line number is in fact incorrect.

Look for other possible errors near that in your code and see if you find anything.