> 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/functions/function-declaration.md).

# 函数的声明与定义

### 结论

函数**不是必须定义在调用之前**，但**必须让编译器在调用之前“见过”它的声明**。

> 一句话：**声明要在调用之前；定义可以放在后面。**

### 1. 定义在前（可以）

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

int add(int a, int b) {   // 定义在调用之前
    return a + b;
}

int main() {
    cout << add(3, 5);    // OK
    return 0;
}
```

### 2. 先声明、后定义（也可以）

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

int add(int a, int b);    // 函数声明（原型），只告诉编译器长什么样

int main() {
    cout << add(3, 5);    // OK，编译器已知道 add 存在
    return 0;
}

int add(int a, int b) {   // 定义放在后面
    return a + b;
}
```

### 3. 什么都没声明就调用（报错）

```cpp
int main() {
    cout << add(3, 5);    // 编译错误：'add' was not declared in this scope
    return 0;
}
int add(int a, int b) { return a + b; }
```

编译到调用处时，编译器还不知道 add 是什么，所以报错。

### 声明 vs 定义

|        | 写法                              | 次数限制             |
| ------ | ------------------------------- | ---------------- |
| 声明（原型） | `int add(int a, int b);`        | 可以多次             |
| 定义     | `int add(int a, int b) { ... }` | 整个程序只能一次（ODR 规则） |

声明可以省略参数名：`int add(int, int);`

### 常见情形

* **递归**：函数在自己的函数体里调用自己没问题，因为函数名在体内已可见。
* **相互调用**（A 调 B、B 又调 A）：至少有一个要先声明，通常两个都前置声明。

  ```cpp
  void b();                 // 前置声明
  void a() { b(); }
  void b() { a(); }
  ```
* **头文件**里一般放**声明**，**定义**放 `.cpp`。
* **类的成员函数**在类内直接写函数体，等于声明+定义，不受前后顺序影响。
* **库函数**：只要 `#include` 了含声明的头文件即可，定义在库里。

### 小结

* 必须：**调用前有声明**。
* 不必须：**定义在调用前**。
* 想省心就把所有函数原型集中写在文件顶部，或先定义再使用。


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