C++ 運算子優先順序列表 –C++ Operator Precedence

這個東西是在http://www.cppreference.com/operator_precedence.html

上的一篇文章，我引用並且大致注釋一下，水平有限。

其實這些說是c++的，對於c語言一樣的適用

The operators at the top of this list are evaluated first.

這些操作符從上倒下優先順序遞減！

Associativity表示執行順序

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   邏輯非 Bitwise complement  按位取反 Pre-increment Pre-decrement Unary minus     負 Unary plus    正 Dereference Address of Cast to a given type 類型轉換 Return size in bytes 返回size大小	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   調用者必須是成員	->* .*	Member pointer selector  成員指標解引用 Member object selector   成員對象解引用	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   左移位 Bitwise shift right  右移位	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接下來3個也是按位	&	Bitwise AND   按位與	flags = flags & 42;	left to right
11	^	Bitwise exclusive OR  邏輯異或	flags = flags ^ 42;	left to right
12	|	Bitwise inclusive (normal) OR  或	flags = flags | 42;	left to right
13接下兩個邏輯	&&	Logical AND  與	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

下面就不翻譯了，大致講對於一些系統不同導致的操作符優先順序的實現不同，副作用吧。

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.