2020-07-08

C/C++ 函式回傳參考用法 function return by reference

這篇主要想講 C++ 函式回傳參考 function return by reference 用法與用途。

class Point {
public:
Point(int _x, int _y) : x(_x), y(_y) {
}

int& x() {
  return x;
}

int& y() {
  return y;
}

private:
  int x = 0;
  int y = 0;
};

用法1. 回傳參考(左值參考 lvalue reference)

point回傳的x或y是回傳參考型別，所以可以直接對它進行修改。

1 2	point.x() = 10; point.y() = 30;

用法2

通常使用這種寫法時，是因為回傳物件的資訊很大，改用回傳參考(reference)會比回傳複本(copy)更有效率。

class Window {
Point& getPosition() { // return reference
  return position;
}
private:
Point position;
}

int main() {
Window w;
Point& pos = w.getPosition();
//Point pos = w.getPosition(); ?? 這樣可以編譯的過？如果可以，那是複本還是參考？

return 0;
}

用法3

C++14 基本上已經支援自動推導型別，使用auto的話編譯器就會自動幫你推導要回傳的類型。

auto& getPosition() {
  return position;
}

int main() {
Window w;
auto& pos = w.getPosition();

return 0;
}

常見問題

不行回傳區域變數

Point& getPosition() {
  Point (3, 5);
  return p; // compile error
}

驗證回傳值參考

這邊做個實驗看看回傳參考的那份，是不是就是本來的那一份
第一個範例
https://www.geeksforgeeks.org/return-by-reference-in-c-with-examples/

參考
Return by reference in C++ with Examples - GeeksforGeeks
https://www.geeksforgeeks.org/return-by-reference-in-c-with-examples/
Returning values by reference in C++ - Tutorialspoint
https://www.tutorialspoint.com/cplusplus/returning_values_by_reference.htm
參考類型函式傳回 | Microsoft Docs
https://docs.microsoft.com/zh-tw/cpp/cpp/reference-type-function-returns?view=vs-2019
傳回值型態
https://openhome.cc/Gossip/CppGossip/returnBy.html
Is the practice of returning a C++ reference variable evil? - Stack Overflow
https://stackoverflow.com/questions/752658/is-the-practice-of-returning-a-c-reference-variable-evil
How to return a class object by reference in C++? - Stack Overflow
https://stackoverflow.com/questions/8914509/how-to-return-a-class-object-by-reference-in-c
reference - How to “return an object” in C++? - Stack Overflow
https://stackoverflow.com/questions/3350385/how-to-return-an-object-in-c