Question

In XCode 4.2, I find that I can write a statement like this:

[obj = obj method];

And there is no error or warning about this.

So, I want to know how the compiler will interpret this? Is this the same as:

obj = [obj method];

Answer 1

Turn up your warning levels. Clang will emit a warning (-Wself-assign):

warning: explicitly assigning a variable of type 'NSObject *' to itself [-Wself-assign]
                  [obj = obj method];

It is not the same as obj = [obj method];. It is interpreted as:

obj = obj;
[obj method];

and the compiler can just omit the self-assign. That is, this program prints a:

NSString * a = @"a";
NSString * b = @"b";
NSLog(@"%@", b = a);

…but why would you write a program using this exact statement? It will be confusing to interpret or just awkward, for many people.