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更正有符号整数左移的相关未定义行为描述 #75

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更正有符号整数左移的相关未定义行为描述 #75

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11 changes: 7 additions & 4 deletions docs/undef.md
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Original file line number Diff line number Diff line change
Expand Up @@ -327,12 +327,15 @@ using T1 = decltype(c << 1); // int

---

对于有符号整数，左移还不得破坏符号位
C++20 前，有符号整数负值不能左移，而左移正值不能导致结果超出对应的无符号整数类型范围。

```cpp
int i = 0;
i << 1; // 可以
i << 31; // 错！
int i = 1;
i << 31; // 可以
int j = 42;
j << 31; // C++20 前错！ C++20 起可以
int k = -1;
k << 0; // C++20 前错！ C++20 起可以
unsigned int u = 0;
u << 31; // 可以
```
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