2021-08-16

C++ lhs 與 rhs 是什麼？

本篇 ShengYu 介紹 C++ 裡 lhs 與 rhs 是什麼？

那我們就開始吧！

看到參數名稱有 lhs 跟 rhs，lhs 全名是 left-hand side 指的是左側，想成等號的左側，同理，rhs 全名是 right-hand side 指的是右側，想成等號的右側，

以下舉個例子來說明 lhs 跟 rhs 的差異，

// g++ lhs-rhs.cpp -o a.out -std=c++11
#include <iostream>
using namespace std;

class Size {
public:
    Size() {}
    Size(int w, int h) : width(w),  height(h)  {}
    
    int width  = 0;
    int height = 0;
};

inline bool operator==(const Size& lhs, const Size& rhs) {
    cout << "lhs=" << lhs.width << "," << lhs.height << "\n";
    cout << "rhs=" << rhs.width << "," << rhs.height << "\n";
    return lhs.width == rhs.width && lhs.height == rhs.height;
}

int main() {
    Size s1;
    Size s2(10, 20);
    
    if (s1 == s2) {
        cout << "equal\n";
    } else {
        cout << "not equal\n";
    }

    return 0;
}

這邊寫個 Size 的 class，並且加上 operator== 可以比較兩個 Size 內容是否相同的功能，
所以 if (s1 == s2) 判斷式裡，s1 就是 lhs，s2 就是 rhs，
我們可以在 operator== 函式裡印出來就知道，結果如下，

1
2
3

lhs=0,0
rhs=10,20
not equal

這次我們加上 operator+= 來試試，使用 operator+= 來做 Size 的累加，

// g++ lhs-rhs-2.cpp -o a.out -std=c++11
#include <iostream>
using namespace std;

class Size {
public:
    Size() {}
    Size(int w, int h) : width(w),  height(h)  {}
    
    int width  = 0;
    int height = 0;
};

inline bool operator==(const Size& lhs, const Size& rhs) {
    cout << "lhs=" << lhs.width << "," << lhs.height << "\n";
    cout << "rhs=" << rhs.width << "," << rhs.height << "\n";
    return lhs.width == rhs.width && lhs.height == rhs.height;
}

inline Size& operator+=(Size& lhs, const Size& rhs) {
    cout << "lhs=" << lhs.width << "," << lhs.height << "\n";
    cout << "rhs=" << rhs.width << "," << rhs.height << "\n";
    lhs.width  += rhs.width;
    lhs.height += rhs.height;
    return lhs;
}

int main() {
    Size s1;
    Size s2(10, 20);
    
    s1 += s2;
    cout << "s1=" << s1.width << "," << s1.height << "\n";

    Size s3(20, 40);
    s3 += s2;
    cout << "s3=" << s3.width << "," << s3.height << "\n";

    return 0;
}

所以在 s1 += s2 運算式裡，，s1 就是 lhs，s2 就是 rhs，
而在 s3 += s2 運算式裡，，s3 就是 lhs，s2 就是 rhs，
我們可以印出結果試試，結果如下，

lhs=0,0
rhs=10,20
s1=10,20
lhs=20,40
rhs=10,20
s3=30,60

其他參考
opencv/modules/gapi/include/opencv2/gapi/own/types.hpp
https://github.com/opencv/opencv/blob/b3db37b99de4d2dcb8c4d8c7568029b3e06893b2/modules/gapi/include/opencv2/gapi/own/types.hpp