l-value與r-value
l-value = r-value;
在等號左邊的叫l-value，在等號右邊的叫r-value。
區別方式
- 有名字一律為左值l-value，沒有名字一律為右值r-value
- 可以取得記憶體位址一律為左值，沒有辦法取得記憶體位址一律為右值
l-value
lvalue simply means an object that has an identifiable location in memory
可以放在等號左邊，並且可以被指派值都是l-value。
等號左邊(l-value)，也就是一個能夠擺在等號左邊的東西∶一個變數，而非常數。
以下都是lavlue:
有定義資料型態(int, double, float, char, long long …)的變數，可以指派值。
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// i is l-value
int i;
//j is l-value
int j = 10; // 10 is r-value
有定義資料型態(int, double, float, char, long long …)的指標，可以指向記憶體位址。
Prerequisites:
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//p1 is l-value
int* p1;
//p2 is l-value
int* p2 = &j; //&j is r-value
可以用*取值運算子修改指標指向的記憶體位址的內容
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//j is l-value
int j = 10; // 10 is r-value
//p2 is l-value
int* p2 = &j; //&j is r-value
//*p2 is lavlue
*p2 = 100; //100 is r-value
可以將變數或指標，指派給&參考
Prerequisites:
參考變數
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// i is l-value
int i = 10; // 10 is r-value
// ref is l-value
int& ref = i;
參考指標
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// i is l-value
int i = 10; // 10 is r-value
//宣告指標
// ptr_i is l-value
int* ptr_i = &i; // &i is r-value
//宣告參考
// 指標指派給參考
// ref_to_ptr is l-value
int*& ref_to_ptr = ptr_i; // ptr_i is l-value
陣列
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// arr is l-value
int arr[] = {0,1,2,3}; // 0,1,2,3 is r-value
陣列元素
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int main() {
// str is l-value
char str[6];
// str[0] is l-value
str[0] = 'H'; // H is r-value
// str[1] is l-value
str[1] = 'e'; // e is r-value
// str[2] is l-value
str[2] = 'l'; // l is r-value
// str[3] is l-value
str[3] = 'l'; // l is r-value
// str[4] is l-value
str[4] = 'o'; // o is r-value
// str[5] is l-value
str[5] = '\0'; // \0 is r-value
cout << str << endl;
return 0;
}
Hello
*(陣列名 + 索引)
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int main() {
// str is l-value
char str[6];
//*(str + 0) is l-value
*(str + 0) = 'H'; // H is r-value
//*(str + 1) is l-value
*(str + 1) = 'E'; // E is r-value
//*(str + 2) is l-value
*(str + 2) = 'L'; // L is r-value
//*(str + 3) is l-value
*(str + 3) = 'L'; // L is r-value
//*(str + 4) is l-value
*(str + 4) = 'O'; // O is r-value
//*(str + 5) is l-value
*(str + 5) = '\0'; // \0 is r-value
cout << str << endl;
return 0;
}
HELLO
指標指向陣列
Prerequisites:
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// array is l-value
int array[5];
// p is l-value
int* p = array; // array is l-value
const與指標
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//var1 is l-value
int var1 = 10; // 10 is r-value
// p is l-value
const int* p = &var1; // &var1 is r-value
r-value
r-value” refers to data value that is stored at some address in memory.
等號右邊的東西可以是字串常數、表達式。
以下是r-value:
字串常數
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int main() {
// cstr2 is l-value
char cstr2[] = "hello"; // hello is r-value
//cstr3 is l-value
char cstr3[6] = "hello";// hello is r-value
//cstr4 is l-value
char cstr4[] = {"hello"};// hello is r-value
//cstr5 is l-value
char cstr5[6] = {"hello"};// hello is r-value
//cstr6 is l-value
char cstr6[6] {"hello"};// hello is r-value
return 0;
}
&陣列 與 &陣列[索引]
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int main() {
// array is l-value
int array[5];
// array is l-value
cout << "陣列名 = " << array << endl;
// &array is r-value
cout << "陣列名地址 = " << &array << endl;
// &array[0] is r-value
cout << "array[0]地址 = " << &array[0] << endl;
// &array[1] is r-value
cout << "array[1]地址 = " << &array[1] << endl;
// &array[2] is r-value
cout << "array[2]地址 = " << &array[2] << endl;
// &array[3] is r-value
cout << "array[3]地址 = " << &array[3] << endl;
return 0;
}
指標 + 整數(0,1,2,3 …)
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int main() {
// array is l-value
int array[5];
// p is l-value
int* p = array; // array is l-value
// p is l-value
cout << "p指標內容 = " << p << endl;
//p + 0 is r-value
cout << "p指標+0 = " << p + 0 << endl;
//p + 1 is r-value
cout << "p指標+1 = " << p + 1 << endl;
//p + 2 is r-value
cout << "p指標+2 = " << p + 2 << endl;
//p + 3 is r-value
cout << "p指標+3 = " << p + 3 << endl;
return 0;
}
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int main() {
// array is l-value
int array[5];
// p is l-value
int* p = array; // array is l-value
// p is l-value
p = p + 1; // p + 1 is r-value
return 0;
}
函式傳回值為物件的值
以下的程式碼，main()和getStudente()都各別有存放傳回值student的記憶體位址，待getStudent()的student變數傳回給main()的student變數後，getStudent()的student變數的記憶體位址就會被釋放。
被記憶體釋放，不會被保留住的物件，也是右值r-value。
傳回物件
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Student getStudent() {
Student student;
return student;
}
int main() {
Student student = getStudent();
return 0;
}
傳回值是臨時物件
臨時物件是右值。
臨時物件建立語法
類別名()
Student()
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Student getStudent() {
return Student();
}
int main() {
Student student = getStudent();
return 0;
}