#pragma

#pragma

01 March 2012 ⌘ Permalink

In iOS source examples, you’ve probably come across the use of #pragma. Pragmas are directives which instruct the compiler to use pragmatic or implementation-dependent features. Xcode supports some of helpful pragmas that can help you organize and manage your code.

#pragma mark A Heading

The mark argument is probably the most common one you’ll see used in Xcode. When added to your code it will give you a nice heading in the navigation bar as shown here:

A mark with a dash (-) such as this:

#pragma mark - A Heading

will include a divider line above the heading. Leave the dash out and you’ll just get the heading. You can also do something like this:

#pragma mark -
#pragma mark A Heading

and you’ll get the same thing.

#pragma unused(var)

You should keep you code clean and ideally without unused variable but placing a #pragma with the unused argument after a variable lets you suppress those “unused variable” warnings you see far too often.

BOOL test = YES;
#pragma unused(test)

#pragma clang diagnostic

(thanks to Guy Shaviv from tvistudio.com for sending this in)

clang diagnostic is useful to get rid of those pesky warnings when performing a selector with ARC from a variable, and ARC doesn’t know if the selector returns a retained object or not.

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Warc-performSelector-leaks"
           [obj performSelector:sel withObject:arg];
#pragma clang diagnostic pop

Some other directives

While I’ve got your attention there two other useful directives you might want to try: #warning and #error. As you can guess, these generate warnings and error durring compile. Warning as especially useful for TODO items. I used to use a TODO comment to remind myself of TODO’s in the code:

// TODO fix this.

but more often than not they were forgotten until moths later when a bug poped up and the line above read ‘// TODO fix this’. Instead, I use a #warning so that all the TODO’s are in the compiler warnings:

#warning This is not complete

which generates:

The #error directive does the same only it’s generates an error and halts the compile. This is useful in the case of a preprocessor conditional

#ifdef SOMETHING
    // Do stuff
#else
    #error SOMETHING was not defined!!!
#endif

If you know of any other neat #pragmas let me know and I’ll add them here.