> For the complete documentation index, see [llms.txt](https://docs.1l.lol/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.1l.lol/basics/headers.md).

# 头文件

### 头文件的作用

`#include` 把库函数声明引入程序。写在文件开头。

### 常用头文件

| 头文件                 | 内容                         |
| ------------------- | -------------------------- |
| `<iostream>`        | cin / cout                 |
| `<cstdio>`          | scanf / printf             |
| `<cstring>`         | strlen / strcpy 等          |
| `<cmath>`           | sqrt / pow / abs 等         |
| `<algorithm>`       | sort / max / min / reverse |
| `<string>`          | string 类型                  |
| `<stack>` `<queue>` | 栈 / 队列                     |
| `<vector>`          | 动态数组                       |

### 万能头文件

```cpp
#include <bits/stdc++.h>
using namespace std;

int main() {
    cout << "Hello" << endl;
    return 0;
}
```

`bits/stdc++.h` 一次包含几乎所有标准库，写竞赛代码很方便；但**编译更慢**、不是标准头文件，部分环境不支持，工程/考试中建议按需包含。

### using namespace std

`using namespace std;` 让我们可以直接写 `cout`，而不必写 `std::cout`。

### 注意

* `#` 开头的都是预处理指令，不以分号结尾。
* 头文件写在所有代码之前。


---

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