Operator Precedence
The precedence order of the operators covered in these notes is important in the evaluation of compound expressions. A compound expression consists of several sub-expressions. Different orders of evaluation are possible in such expressions. That is, the results of evaluating the entire compound expression depend on the order of evaluating its sub-expressions.
To ensure unique evaluation of all compound expressions, C++ defines rules of precedence on the operators in a compound expression. This order is from top to bottom in the table shown below. The operators associate operands in each sub-expression as noted in the right column, mostly from left to right.
| Operator | Associate From |
|---|---|
:: | Left to right |
[ ] . -> ++ (postfix) -- (postfix) | Left to right |
++ (prefix) -- (prefix) + - & ! (all unary) new new[] delete delete[] (type), type() | Right to left |
.* ->* | Left to right |
* / % | Left to right |
+ - | Left to right |
>> << | left to right |
< <= > >= | Left to right |
== != | Left to right |
&& | Left to right |
| || | Left to right |
= += -= *= /= %= | Right to left |
?: | Left to right |
, | Left to right |
The scope resolution operator :: has the highest precedence. An expression containing this operator is always evaluated first.
We can change the order of evaluation within a compound expression by enclosing a sub-expression in parentheses. That is, we use (sub-expression) to evaluate sub-expression before applying the rules of precedence to the entire expression.
For example,
2 + 3 * 5 evaluates to 2 + 15 which evaluates to 17.
(2 + 3) * 5 evaluates to 5 * 5 which evaluates to 25.