?: in Objective-C [duplicate]

Question

In this iOS tutorial, there is a line of code with a ? followed by a :. In the context of the comment for the code, I thought it was some sort of ternary operation, however, that's obviously not the syntax for a ternary operator. Is there a name for what is happening in this code with the ?:?

// Initialize the list of weather items if it doesn't exist
NSMutableArray *array = self.xmlWeather[@"weather"] ?: [NSMutableArray array];

Answer 1

It's a GCC extension:

6.7 Conditionals with Omitted Operands

The middle operand in a conditional expression may be omitted. Then if the first operand is nonzero, its value is the value of the conditional expression.

Therefore, the expression

x ? : y

has the value of x if that is nonzero; otherwise, the value of y.

This example is perfectly equivalent to

x ? x : y

In this simple case, the ability to omit the middle operand is not especially useful. When it becomes useful is when the first operand does, or may (if it is a macro argument), contain a side effect. Then repeating the operand in the middle would perform the side effect twice. Omitting the middle operand uses the value already computed without the undesirable effects of recomputing it.