What's a 'null defined macro'?

Question

I'm learning objective-C and Cocoa. In the Apple tutorial I'm working through there's a side note that says:

IBOutlet is a null-defined macro, which the C preprocessor removes at compile time.

I'm curious - what's a null-defined macro?

Cheers Ben

Answer 1

#define IBOutlet

Whenever IBOutlet is used in program text, it will be replaced with nothing at all.

Answer 2

FYI, in this particular case, the reason the IBOutlet even exists is simply so that Interface Builder can parse the source file and glean bits of understanding from it. It's a clue (well, a bit stronger than a clue) that the variable preceded by IBOutlet should show up as an Outlet in Interface Builder when designing your UIs.

Answer 3

A null-defined macro is a macro which will be replaced by nothing (will be removed) by the preprocessor. It's role is to give a hint about something in code, such as:

#define IN
#define OUT
#define INOUT

int myFunction(IN char *name, INOUT char *address, OUT char *phone);

This declaration suggests that name is a input variable for the function, address is both input and output, phone is an output variable.

Answer 4

Also - if you're unsure how anything is defined - command double-click it and Xcode will open the definition in the original source file.

Answer 5

Oh and while I'm at it. Option double click will (attempt to) open up the documentation for the double clicked symbol.