C Traps：优先级常见错误
逻辑与关系运算符
if (flags & FLAG != 0) {...}
这类错误以前也犯过，因为!=的优先级比&要高所以实际上是这样的
if (flags & (FLAG != 0))
可以把&、|也作为一种逻辑操作，而逻辑操作符的优先级总是比比较操作符、运算操作符来得低。可以这样想，首先进行了一些运算，然后才比较他们的值，通过比较有了布尔值，最后就进行逻辑与、或操作了。
位移与算术运算符
上面的错误可能会在不清楚的情况下使用括号，但是下面的情况则很难会觉察到，记得好几次都是莫名其妙的位移操作出现过问题。
res = hi << 4 + low
实际上相当于：
res = hi << (4 + low)
这种编码错误个人感觉非常难以察觉，尤其在对优先级生疏的情况下。类似的有
res = low + hi << 4
相当于
res = (low + hi) << 4
赋值与比较运算符
while ((c=getc(in) != EOF) {...}
内层的括号是不可缺少的，因为赋值运算优先级低于比较运算符。
虽然有时码农编码是重复劳动，但是码农的基本功还是需要努力修炼的。
给出一个C++的优先级表
| Precedence | Operator | Description | Associativity |
|---|---|---|---|
| 1 | :: |
Scope resolution | Left-to-right |
| 2 | ++ -- |
Suffix/postfix increment and decrement | |
() |
Function call | ||
[] |
Array subscripting | ||
. |
Element selection by reference | ||
-> |
Element selection through pointer | ||
| 3 | ++ -- |
Prefix increment and decrement | Right-to-left |
+ − |
Unary plus and minus | ||
! ~ |
Logical NOT and bitwise NOT | ||
(type) |
Type cast | ||
* |
Indirection (dereference) | ||
& |
Address-of | ||
sizeof |
Size-of | ||
new, new[] |
Dynamic memory allocation | ||
delete, delete[] |
Dynamic memory deallocation | ||
| 4 | .* ->* |
Pointer to member | Left-to-right |
| 5 | * / % |
Multiplication, division, and remainder | |
| 6 | + − |
Addition and subtraction | |
| 7 | << >> |
Bitwise left shift and right shift | |
| 8 | < <= |
For relational operators < and ≤ respectively | |
> >= |
For relational operators > and ≥ respectively | ||
| 9 | == != |
For relational = and ≠ respectively | |
| 10 | & |
Bitwise AND | |
| 11 | ^ |
Bitwise XOR (exclusive or) | |
| 12 | | |
Bitwise OR (inclusive or) | |
| 13 | && |
Logical AND | |
| 14 | || |
Logical OR | |
| 15 | ?: |
Ternary conditional | Right-to-left |
= |
Direct assignment (provided by default for C++ classes) | ||
+= −= |
Assignment by sum and difference | ||
*= /= %= |
Assignment by product, quotient, and remainder | ||
<<= >>= |
Assignment by bitwise left shift and right shift | ||
&= ^= |= |
Assignment by bitwise AND, XOR, and OR | ||
| 16 | throw |
Throw operator (for exceptions) | |
| 17 | , |
Comma | Left-to-right |
记忆：
1. 获得要进行操作的数（()函数返回值或括号表达式值，[]数组下表，. ->成员引用，从左向右结合：a->b->c->d合法）
2. 进行单目操作（自增，自减，取地址，解引用，sizeof等，从右向左结合：*p++等同于*(p++)）
3. 进行双目操作（乘除模，加减，位移，关系，位逻辑，逻辑，逻辑运算优先级低于其他关系算术运算，或低于与，从左向右结合）
4. 赋值（从右向左结合：a=b=c）
5. 逗号表达式（从左向右结合）
() 函数调用、括号肯定可以记住
参考：
1. C traps and pitfalls
2. http://en.cppreference.com/w/cpp/language/operator_precedence
