Thursday, March 15, 2012

Objective-C: ternary operator

The ternary operator, also known as a conditional expression is a C construct.  What follows is an example used for conditional assignment:

result = condition_expression ? true_expression : false_expression;

If condition_expression evaluates to true, result will be assigned the true_expression; otherwise, result will be assigned the false_expression.  A GNU extension to the ternary operator, also available in Objective-C, is the ability to omit the true_expression as follows:

result = first_expression ?: second_expression;

result will be assigned the value of first_expression if it evaluates to true or second_expression if first_expression evaluates to false.

nil in Objective-C evaluates to false, so the GNU extension becomes particularly useful for ensuring default values during assignment, which is a pattern used widely in JavaScript.

A contrived example might be an init method that takes an NSDictionary with options for configuring the instance, as follows:

- (void)initWithOptions:(NSDictionary *)options {
	self = [super init];
	
	_options = options ?: kDefaultOptions;

	return self;
}

The assignment ensures _options is not null.  options will be assigned to the _options ivar if non-nil; otherwise _options will be assigned the kDefaultOptions dictionary.

Another awesome use case is for caching values, rather than initialize them all in the init method.

@implementation MyObj {
	NSString *_someValue;
}

@property (nonatomic, strong, readonly) NSString *someValue;

@synthesize someValue=_someValue;

- (id)init {
	…
}

- (NSString *)someValue {
	return _someValue ?: (_someValue = [some expensive operation]);
}

When a consumer accesses the someValue property of MyObj, if _someValue is nil, the right size of the ternary expression is returned, which retrieves the value via some expensive operation and stores it in _someValue.

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