c - 基本 C 位运算示例

Question

int sampleVariable; // declared and initialized and used elsewhere

if (sampleVariable & 2)
     someCodeIwantExecuted();

因此，如果我想手动操作 sampleVariable 以便 if 语句评估为 true 并执行 someCodeIwantExecuted() 我会执行以下操作？

sampleVariable |= (1 << 1);

请记住，我不知道 sampleVariable 的值是什么，我想保持其余位相同。只需更改该位，以便 if 语句始终为真。

Answer 1

解决方案相当直接。

//  OP suggestion works OK
sampleVariable |= (1 << 1);

// @Adam Liss rightly suggests since OP uses 2 in test, use 2 here.
sampleVariable |= 2

// My recommendation: avoid naked Magic Numbers
#define ExecuteMask (2u)
sampleVariable |= ExecuteMask;
...
if (sampleVariable & ExecuteMask)

---

注意：在使用 shift 样式时(1 << 1)，请确保类型与1您的目标类型匹配

unsigned long long x;
x |= 1 << 60;  // May not work if `sizeof(int)` < `sizeof(x)`.
x |= 1ull << 60;

进一步：考虑unsigned类型的优点。

// Assume sizeof int/unsigned is 4.
int i;
y |= 1 << 31;  // Not well defined
unsigned u;
u |= 1u << 31;  // Well defined in C.