l-value參考與r-value參考
Prerequisites:
l-value參考
l-value參考指向變數
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int main() {
int i = 10;
int& l_ref = i;
cout << "l-value ref = " << l_ref << endl;
cout << "=== modify ref === " << endl;
l_ref = 55;
cout << "l-value ref = " << l_ref << endl;
return 0;
}
l-value ref = 10
=== modify ref ===
l-value ref = 55
r-value參考
幫右值取名字
語法
右值資料型態&& 右值變數名 = 右值;
int&& r_ref = 100;
r-value參考指向數字
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int main() {
int&& r_ref = 100;
cout << "r-value ref = " << r_ref << endl;
cout << "=== modify ref === " << endl;
r_ref = 1000;
cout << "r-value ref = " << r_ref << endl;
return 0;
}
r-value ref = 100
=== modify ref ===
r-value ref = 1000
取出右值記憶體位址
右值取名字後，名字就變成左值，就可以取出記憶體位址。
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int main() {
int&& r = 10;
cout << "r address = " << &r << endl;
return 0;
}
r address = 0x7ff7bfeff45c
修改右值
右值取名字後，名字就變成左值，可以修改值。
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int main() {
//r-value
int&& r = 10;
cout << "Before r = " << r << endl;
r = 20;
cout << "After r = " << r << endl;
Before r = 10
After r = 20
左值不能放在右值參考
以下程式碼會編譯錯誤
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int main() {
//l_val是l-value
int l_val = 10;
//定義右值參考
int&& r_val_ref = l_val;
return 0;
}
右值不能放在左值參考
以下程式碼會編譯錯誤
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int main() {
// 10 is r-value
int& l_val_ref = 10;
return 0;
}
函式參數為l-value參考
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#include <iostream>
using namespace std;
void func(int& l_ref) {
l_ref = 20;
}
int main() {
int i = 10;
cout << "before i = " << i << endl;
func(i);
cout << "after i = " << i << endl;
return 0;
}
before i = 10
after i = 20
函式參數為r-value參考
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#include <iostream>
using namespace std;
void func(int&& r_ref) {
cout << "r-value = " << r_ref << endl;
}
int main() {
func(1000);
return 0;
}
r-value = 1000
引數與參數r-value參考資料型態不同
引數與參數的資料型態不同，若自動轉型可以轉成功，也可以使用r-value參考。
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void func1(int&& r_ref) {
cout << "r-value = " << r_ref << endl;
}
int main() {
func1('B');
return 0;
}
r-value = 66
函式傳回值為右值
語法
類別名&& 變數名 = 函式();
Student&& s1 = getStudent1();
把右值取了名字，這個名字就是左值，可以做左值能做的事，比如存取物件成員變數，呼叫成員函式。
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#include <iostream>
using namespace std;
class Student {
public:
string name;
};
Student getStudent1() {
Student s;
s.name = "Bill";
return s;
}
Student getStudent2() {
//臨時物件
return Student();
}
int main() {
Student&& s1 = getStudent1();
cout << "s1 name = " << s1.name << endl;
Student&& s2 = getStudent2();
s2.name = "Mary";
cout << "s2 name = " << s2.name << endl;
return 0;
}
s1 name = Bill
s2 name = Mary