Question

So I’m trying to write an Objective-C macro that takes an object and a value, releases the original contents, and makes the assignment. Ideally, it would look like this:

myObj = RELEASE_AND_ASSIGN([SomeObject object])

Some things to note, if we leave out the assignment and put the object in the macro, the macro is easily written like so:

#define RELEASE_AND_ASSIGN_TO(obj, expr) [obj release]; obj = expr;

The reason why I don’t want it that way is I feel that reading the code is tougher. I want my eyes to see the assignment on the left. Having something akin to a function call in the place of an assignment I think is awful for clean code and readability.

I tried one hybrid possibility:

#define RELEASE_AND_ASSIGN_TO(obj, expr) [obj release], obj = expr;

The idea is that the release and assignment occurs, and the comma operator returns obj. Much to my confusion, it seems that Objective-C wants to use the [obj release]’s return value. I don’t know why, I thought the right side was taken?

I’m a little lost. Right now, not having the assignment made explicit when calling the macro I don’t think is a style I want to advocate. Any advice?

Answer 1

I don’t know what the heck you are trying to do, but the comma operator takes the right expression, yes, but then you must also put ( and ) around it, for else the statement

myObj = [obj release], obj = expr;

will just be

myObj = [obj release]; obj = expr;

but

myObj = ([obj release], obj = expr);

will be

[obj release];
myObj = obj = expr;

So try it like this:

#define RELEASE_AND_ASSIGN_TO(obj, expr) ([(obj) release], (obj) = (expr))