C++ 运算符优先级列表 --C++ Operator Precedence

这个东西是在http://www.cppreference.com/operator_precedence.html

上的一篇文章，我引用并且大致注释一下，水平有限。

其实这些说是c++的，对于c语言一样的适用

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

这些操作符从上倒下优先级递减！

Associativity表示执行顺序

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 逻辑非 Bitwise complement 按位取反 Pre-increment Pre-decrement Unary minus 负 Unary plus 正 Dereference Address of Cast to a given type 类型转换 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 调用者必须是成员	->* .*	Member pointer selector 成员指针解引用 Member object selector 成员对象解引用	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接下来3个也是按位	&	Bitwise AND按位与	flags = flags & 42;	left to right
11	^	Bitwise exclusive OR 逻辑异或	flags = flags ^ 42;	left to right
12	|	Bitwise inclusive (normal) OR 或	flags = flags | 42;	left to right
13接下两个逻辑	&&	Logical AND 与	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

下面就不翻译了，大致讲对于一些系统不同导致的操作符优先级的实现不同，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 <it></it>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, <it></it>x could take a different value.

Furthermore, while ++x evaluates to x+1, the side effect of actually storing that new value in <it></it>x could happen at different times, resulting in different values for <it></it>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.