位运算的一个坑

Tue, 30 Jul 2013 11:32:30 +0800

来看这段代码

#include <stdio.h>

int main() {
    int a = 1, b = 0;
    if (a|b == 0) {
        printf("1\n");
    } else {
        printf("2\n");
    }
    return 0;
}

这段代码想表达的意思是，如果两个 int 都为 0，那么走 if 分支，不然的话，走 else 分支，嗯，大家都是这么想的，实际上，他输出什么呢？

输出 1。

因为位运算的优先级比判等的优先级低。

http://en.cppreference.com/w/cpp/language/operator_precedence

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

位运算 on ZRJ | 学习笔记

位运算的一个坑