The operators at the top of this list are evaluated first.
| Precedence | Operator | Description | Example | Associativity |
|---|---|---|---|---|
| 1 | :: | Scoping operator | Class::age = 2; | none |
| 2 | () [] -> . ++ -- | Grouping operator Array access Member access from a pointer Member access from an object Post-increment Post-decrement | (a + b) / 4; array[4] = 2; ptr->age = 34; obj.age = 34; for( i = 0; i < 10; i++ ) ... for( i = 10; i > 0; i-- ) ... | left to right |
| 3 | ! ~ ++ -- - + * & (type) sizeof | Logical negation Bitwise complement Pre-increment Pre-decrement Unary minus Unary plus Dereference Address of Cast to a given type Return size in bytes | if( !done ) ... flags = ~flags; for( i = 0; i < 10; ++i ) ... for( i = 10; i > 0; --i ) ... int i = -1; int i = +1; data = *ptr; address = &obj; int i = (int) floatNum; int size = sizeof(floatNum); | right to left |
| 4 | ->* .* | Member pointer selector Member object selector | ptr->*var = 24; obj.*var = 24; | left to right |
| 5 | * / % | Multiplication Division Modulus | int i = 2 * 4; float f = 10 / 3; int rem = 4 % 3; | left to right |
| 6 | + - | Addition Subtraction | int i = 2 + 3; int i = 5 - 1; | left to right |
| 7 | << >> | Bitwise shift left Bitwise shift right | int flags = 33 << 1; int flags = 33 >> 1; | left to right |
| 8 | < <= > >= | Comparison less-than Comparison less-than-or-equal-to Comparison greater-than Comparison geater-than-or-equal-to | if( i < 42 ) ... if( i <= 42 ) ... if( i > 42 ) ... if( i >= 42 ) ... | left to right |
| 9 | == != | Comparison equal-to Comparison not-equal-to | if( i == 42 ) ... if( i != 42 ) ... | left to right |
| 10 | & | Bitwise AND | flags = flags & 42; | left to right |
| 11 | ^ | Bitwise exclusive OR | flags = flags ^ 42; | left to right |
| 12 | | | Bitwise inclusive (normal) OR | flags = flags | 42; | left to right |
| 13 | && | Logical AND | if( conditionA && conditionB ) ... | left to right |
| 14 | || | Logical OR | if( conditionA || conditionB ) ... | left to right |
| 15 | ? : | Ternary conditional (if-then-else) | int i = (a > b) ? a : b; | right to left |
| 16 | = += -= *= /= %= &= ^= |= <<= >>= | Assignment operator Increment and assign Decrement and assign Multiply and assign Divide and assign Modulo and assign Bitwise AND and assign Bitwise exclusive OR and assign Bitwise inclusive (normal) OR and assign Bitwise shift left and assign Bitwise shift right and assign | int a = b; a += 3; b -= 4; a *= 5; a /= 2; a %= 3; flags &= new_flags; flags ^= new_flags; flags |= new_flags; flags <<= 2; flags >>= 2; | right to left |
| 17 | , | Sequential evaluation operator | for( i = 0, j = 0; i < 10; i++, j++ ) ... | left to right |
One important aspect of C++ that is related to operator precedence is the order of evaluation and the order of side effects in expressions. In some circumstances, the order in which things happen is not defined. For example, consider the following code:
float x = 1; x = x / ++x;
The value of x is not guaranteed to be consistent across different compilers, because it is not clear whether the computer should evaluate the left or the right side of the division first. Depending on which side is evaluated first, x could take a different value.
Furthermore, while ++x evaluates to x+1, the side effect of actually storing that new value in x could happen at different times, resulting in different values for x.
The bottom line is that expressions like the one above are horribly ambiguous and should be avoided at all costs. When in doubt, break a single ambiguous expression into multiple expressions to ensure that the order of evaluation is correct