# Header檔

## 什麼是 Header (.h) 檔？

Header (.h) 檔是一個專門儲存function、class、structure等等定義的檔案。這讓程式可以先在 Header 檔內定義功能，並在 Source Code (.cpp) 裡實踐. 這樣可以把程式分割成多個檔案，減少每個檔案的行數並增加可維護性。

## 基本範例

```cpp
// library.h
int fibonacci(int n) {
    int s = 0;
    int d = 1;
    for (int i = 0; i < n-1; i++) {
        int tmp = d;
        d = s + d;
        s = tmp;
    }
    return d;
}
```

```cpp
#include "library.h"

// main.cpp
int main() {
    printf("%d\n", fibonacci(5));
    return 0;
}
```

## 分割定義與實踐

假設我們需要一個程式，幫我執行把degree轉radian，或是把任何角度轉換成-180\~180的小計算，但是我又不想讓 .h 檔變很複雜。這時可以用以下的方法

```cpp
// include/utils.h

namespace Utils {
    float deg_to_rad(float degrees);
    float rad_to_deg(float radians);
    float format_angle(float degrees);
}
```

```cpp
// include/main.h

#include "utils.h"
```

```cpp
// src/utils.cpp
#include "main.h"

float Utils::deg_to_rad(float degrees) {
    return degrees * M_PI / 180.0;
}

float Utils::rad_to_deg(float degrees) {
    return degrees * 180.0 / M_PI;
}

float Utils::format_angle(float degrees) {
    int sign = a < 0 ? -1 : 1;
    float positive_a = abs(a);
    float mod = std::fmod(positive_a, 360);
    if (mod <= 180) {
        return sign * mod;
    } 
    else {
        return sign * (mod - 360);
    }
}
```

接下來就可以在任何有 include main.h 的地方使用 Utils 了

```cpp

// main.cpp

void opcontrol() {
    printf("%f\n", Utils::deg_to_rad(180));
}
```


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