Objective-C Default Argument Value

Question

Hey there, quick question here. I\'m sure there\'s a simple answer.

Coming from PHP, I\'m used to declaring a function with a default argument value like this:

Answer 1

There are two standard patterns for achieving what you want.

(1) write a many argument form of a method and then provide fewer argument convenience versions. For example, consider the following methods on NSString:

- (NSComparisonResult)compare:(NSString *)string;
- (NSComparisonResult)compare:(NSString *)string options:(NSStringCompareOptions)mask;
- (NSComparisonResult)compare:(NSString *)string options:(NSStringCompareOptions)mask
            range:(NSRange)compareRange;
- (NSComparisonResult)compare:(NSString *)string options:(NSStringCompareOptions)mask
            range:(NSRange)compareRange locale:(id)locale;

The first three are conceptually [and likely concretely, I didn't check] implemented as calls through to the fourth version. That, is -compare: calls -compare:options:range:locale: with appropriate default values for the three additional arguments.

(2) The other pattern is to implement the many argument version of the method and provide default values when an argument is NULL/nil or set to some value that indicates the default is desired. NSData has methods that are implemented with this pattern. For example:

+ (id)dataWithContentsOfFile:(NSString *)path options:(NSDataReadingOptions)readOptionsMask
            error:(NSError **)errorPtr;

If you pass 0 for the readOptionsMask argument, the NSData will read the contents of the file using an internally defined default configuration. That default configuration may change over time.