What does the question mark and the colon (?: ternary operator) mean in objective-c?

This is the C ternary operator (Objective-C is a superset of C):

label.frame = (inPseudoEditMode) ? kLabelIndentedRect : kLabelRect;

is semantically equivalent to

if(inPseudoEditMode) {
 label.frame = kLabelIndentedRect;
} else {
 label.frame = kLabelRect;
}

The ternary with no first element (e.g. variable ?: anotherVariable) means the same as (valOrVar != 0) ? valOrVar : anotherValOrVar

It's the ternary or conditional operator. It's basic form is:

condition ? valueIfTrue : valueIfFalse

Where the values will only be evaluated if they are chosen.

Simply, the logic would be

(condition) ? {code for YES} : {code for NO}

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