C++ Reference Variable 參考

概念

參考是變數別名，當參考被改變變數的值也會改變。
常常用在傳入函式的參數，因為一般的變數傳入函式中是複製一份去做計算，
但是參考的話是複製位址，因此更改後不會因為離開函式而變回原本的值。
另外，以參考為參數傳入函式，可以避免stack overflow的問題，
因為參考是複製位址，並非複製整個變數。

語法示範

• 以&為開頭表示使用參考，與取址一樣的符號，但在宣告時使用即是代表要做參考
  ex:

    string name = "Tony";
    // 使用參考
    string &nickName = name;
    // 取地址
    cout << &name << endl;

• 使用參考時必須一開始就初始化
• 參考一旦決定了為那個變數的參考後便不能再更改
  ex:

    string name = "Tony";
    string &nickname = name;
    string otherName = "Annie";
    &nickname = otherName; // 錯誤

• const參考與其他變數使用const相同，不能更改
  ex:

    string testName = "test";
    const string &ref = testName;
    testName = "pass";
    cout << testName << endl; // output: pass
    cout << ref << endl;      // output: pass

    // const can't be changed
    ref = "impossible"; // error

• 範例

#include <iostream>
using namespace std;

int main() {
    string name = "Tony";
    string others = "Annie";
    string &nickname = name;

    // reference need to be initialized at beginning
    // error : string &wrong;

    // reference can not change the connection
    // error : &nickname = others;   // because nickname is the reference of name

    // const can not be changed
    string testName = "test";
    const string &ref = testName;
    testName = "pass";
    cout << testName << endl; // output: pass
    cout << ref << endl;      // output: pass

    // const can't be changed
    // error : ref = "impossible";


    cout << "name: " << name << ", address: " << &name <<endl;        // name: Tony, address: 0x7fff59f9e918
    cout << "nickname: " << nickname << ", address: " << &nickname <<endl;    // name: Tony, address: 0x7fff59f9e918

    nickname = "Tom";
    cout << "name: " << name << ", address: " << &name <<endl;        // name: Tom, address: 0x7fff59f9e918
    cout << "nickname: " << nickname << ", address: " << &nickname <<endl;  // name: Tom, address: 0x7fff59f9e918

    name = "James";
    cout << "name: " << name << ", address: " << &name <<endl;         // name: James, address: 0x7fff59f9e918
    cout << "nickname: " << nickname << ", address: " << &nickname <<endl;  // name: James, address: 0x7fff59f9e918
}

應用

#include <iostream>
using namespace std;

void swapNormal(int, int);
void swapRef(int &, int &);

int main() {
     int a = 1000;
    int b = 10;
    cout << "Without any manipulation..." << endl;
    cout << "a: " << a << endl;
    cout << "b: " << b << endl;
    swapNormal(a, b);
    cout << "After swapNormal:" << endl;
    cout << "a : " << a << endl;
    cout << "b : " << b << endl;

    swapRef(a, b);
    cout << "After swapRef:" << endl;
    cout << "a : " << a << endl;
    cout << "b : " << b << endl;
}

void swapNormal(int x, int y) {
    cout << "In swapNormal..." << endl;
    /*
        複製一份
        x = a;
        y = b;
    */
    int tmp;
    tmp = x;
    x = y;
    y = tmp;

    cout << "a: " << x << endl;
    cout << "b: " << y << endl;
}
void swapRef(int &x, int &y) {
    cout << "In swapRef..." << endl;
    /*
        複製一份
        &x = a;
        &y = b;
    */
    int tmp;
    tmp = x;
    x = y;
    y = tmp;

    cout << "a: " << x << endl;
    cout << "b: " << y << endl;
}

output:

Without any manipulation...
a: 1000
b: 10
In swapNormal...
a: 10
b: 1000
After swapNormal:
a : 1000
b : 10
In swapRef...
a: 10
b: 1000
After swapRef:
a : 10
b : 1000

tags: C++

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