> 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/strings/char-array-length.md).

# 字符数组的有效长度

### 字符数组与字符串的区别

* **字符数组** `char s[N]`：以 `'\0'` 结尾，长度固定。
* **string**：C++ 标准库类型，自动管理长度，更安全。

求长度要用对方法：

| 对象      | 方法                         |
| ------- | -------------------------- |
| string  | `s.size()` 或 `s.length()`  |
| char 数组 | `strlen(s)`（需 `<cstring>`） |

> ⚠️ 常见错误：对字符数组用 `size()`（字符数组没有成员函数 size()）。`size()` 只能用于 string。

### strlen 的原理

`strlen` 从首地址开始数，直到遇到 `'\0'` 才停止，所以字符数组**必须有结尾的 \0**。

```cpp
#include <iostream>
#include <cstring>
using namespace std;

int main() {
    char s[20] = "hello";       // 实际内容 h e l l o \0
    cout << strlen(s) << endl;  // 5，不含 '\0'
    cout << sizeof(s) << endl;  // 20，数组总字节数（注意区别！）
    return 0;
}
```

### 手写求长度

```cpp
int len = 0;
while (s[len] != '\0') len++;
```

### 输入与长度

```cpp
char s[100];
cin >> s;              // 遇空格/换行停止，读入一个单词
cin.getline(s, 100);   // 读入一整行（含空格）
cout << strlen(s);
```

### 要点

* `sizeof(s)` 是数组占用的字节数，不是有效字符数。
* 字符数组必须预留 `'\0'` 的位置：存 n 个字符至少开 `n+1`。


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