Objective-C 2.0 Operator Precedence

In the previous chapter of Objective-C 2.0 Essentials we looked at Objective-C operators and expressions. An equally important area to understand is operator precedence. This is essentially the order in which Objective-C evaluates expressions comprising more than one operator.

An Example of Objective-C Operator Precedence

When humans evaluate expressions, they usually do so starting at the left of the expression and working towards the right. For example, working from left to right we get a result of 300 from the following expression:

10 + 20 * 10 = 300

This is because we, as humans, add 10 to 20, resulting in 30 and then multiply that by 10 to arrive at 300. Ask Objective-C to perform the same calculation and you get a very different answer:

#import <Foundation/Foundation.h>

int main (int argc, const char * argv[])
{
        NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];

        int x = 10 + 20 * 10;

        NSLog(@"Result is %i", x);
        [pool drain];
        return 0;
}

When executed, the result of the calculation is assigned to integer variable x and subsequently displayed using the NSLog call:

2009-10-09 10:37:07.856 t[3511] Result is 210

As we can see from the above output, Objective-C considers the answer to be 210. This is a direct result of operator precedence. Objective-C has a set of rules that tell it in which order operators should be evaluated in an expression. Clearly, Objective-C considers the multiplication operator (*) to be of a higher precedence than the addition (+) operator.

Objective-C Operator Precedence and Associativity

When addressing the issue of operator precedence in some scripting and programming languages, all that is generally required is a table listing the operators in order of precedence from highest to lowest. Objective-C has more in common with languages such as Java and C# in that operators are grouped together at different precedence levels. When operators from the same precedence level are found within the context of a single expression, a rule as to the order in which the operators are to be evaluated is followed. This rule is referred to as the associativity and differs from one group to the next. The following table outlines the operator precedence groups and corresponding associativity for Objective-C:

Operator	Description	Precedence	Associativity
[] . () ->	access array element or message expression access object member or method invoke a method or function pointer to structure member	Highest	left to right
++ -- + - ! ~ * & sizeof (type)	increment decrement unary plus unary minus logical NOT ones complement pointer reference address of size of object type cast		right to left
* / %	multiply divide modulus		left to right
+ -	add subtract		left to right
<< >> <	bitwise shift left bitwise shift right less than		left to right
<= >= >	less than or equal to greater than or equal to greater than		left to right
== !=	equality inequality		left to right
&	bitwise AND		left to right
^	bitwise XOR		left to right
|	bitwise OR		left to right
&&	logical AND		left to right
||	logical OR		left to right
?:	conditional		right to left
= += -= *= /=  %= &= ^= |= <<= >>= >>>=	assignment		right to left
,	comma	Lowest	right to left

As we can see from the table, when operators from the same precedence level appear in an expression, the operators are evaluated either left to right or right to left depending on the associativity of that group. For example, the following expression will be evaluated left to right because both operators are in the same precedence level and this is the rule dictated by the corresponding associativity:

int x = 10 * 20 / 5;

Overriding Operator Precedence

The precedence and associativity rules built into Objective-C can be overridden by surrounding the lower priority section of an expression with parentheses. For example, we can override precedence and force a left to right evaluation of our original example as follows:

int x = (10 + 20) * 10;

NSLog(@"Result is %i", x);

In the above example, the expression fragment enclosed in parentheses is evaluated before the higher precedence multiplication resulting in the expression equaling 300 instead of the 210 result we saw when we allowed precedence to take effect:

2009-10-09 11:42:43.845 t[5630] Result is 300