Question

I made Sender class and sender object:

Sender *sender = [[Sender alloc] init];

Then, I wrote test:

// should success
[sender upload:[UIImage imageNamed:@"test2.jpg"] withName:@"test2.jpg"];
// should fail
[sender upload:nil withName:@"test2.jpg"];

Then, I wrote nil check code:

- (void)upload:(UIImage *)image withName:(NSString *)name
{
    if ([image isEqual:nil]) {
        ...
    }
    ...
}

But nil check is ignored.
How can I check whether the image parameter is nil or not?
Thank you for your kindness.

Answer 1

Short answer, replace [image isEqual:nil] with image == nil.

Long answer, when you write:

[image isEqual:nil]

you are asking for the message isEqual: to be sent to the object whose reference is stored in the variable image.

However you don’t wish to send a message to an object, you wish to determine whether the variable image contains a reference to an object or not. To do this you need to compare the value of your reference with the “no reference” value – which is written nil. The value comparison operator in Objective-C (and C et al) is == so you need to test:

image == nil

Now you might ask why did the code you have execute without error, just not producing the result you expected, given that there is no object to send a message to? Objective-C supports sending messages to “no object”, aka nil, and returns the “zero” value for the result type, i.e. 0, nil etc. When converted to a boolean value, as in your if statement, a “zero” value produces NO.

HTH