參考指向指標
Prerequisites:
參考指向指標，代表參考可以像指標一樣，使用*取值運算子，取出原指標指向的記憶體位址中的值，也可以修改原指標指向的記憶體位址中的值，或更改原指標指向的記憶體位址。
宣告參考
資料型態*& 別名 = 原指標;
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int i = 10;
cout << "i address:" << &i << endl;
int j = 100;
cout << "j address:" << &j << endl;
//宣告指標
int* ptr_i = &i;
//宣告參考
// 原指標指派給參考
int*& ptr_ref = ptr_i;
cout << "== Change value ==" << endl;
cout << "Before i value:" << i << endl;
cout << "Before value:" << *ptr_ref << endl;
//修改原指標指向的記憶體位址中的值
*ptr_ref = 20;
cout << "After i value:" << i << endl;
//取出原指標指向的記憶體位址中的值
cout << "After value:" << *ptr_ref << endl;
cout << "== Change address ==" << endl;
cout << "Before address:" << ptr_ref << endl;
//更改原指標指向的記憶體位址
ptr_ref = &j;
cout << "after address:" << ptr_ref << endl;
return 0;
i address:0x7ff7bfeff468
j address:0x7ff7bfeff464
== Change value ==
Before i value:10
Before value:10
After i value:20
After value:20
== Change address ==
Before address:0x7ff7bfeff468
after address:0x7ff7bfeff464
參考以下文章
References to pointers is a modifiable value that’s used same as a normal pointer. (google翻譯)對指標的參考是一個可修改的值，其使用方式與普通指標相同。
函式參數為指向指標的參考
如果要在函式中修改指標指向的記憶體位址，就需要使用指標的指標，也可以使用參考。
引數為指標，函式參數宣告為指標資料型態*& 別名，這樣才可以修改指標指向的記憶體位址。
參考以下文章
https://www.geeksforgeeks.org/passing-reference-to-a-pointer-in-c/
If a pointer is passed to a function as a parameter and tried to be modified then the changes made to the pointer does not reflects back outside that function. This is because only a copy of the pointer is passed to the function. It can be said that “pass by pointer” is passing a pointer by value. In most cases, this does not present a problem. But the problem comes when you modify the pointer inside the function. Instead of modifying the variable, you are only modifying a copy of the pointer and the original pointer remains unmodified.
(google翻譯)如果將指標作為參數傳遞給函式並嘗試對其進行修改，則對指標所做的更改不會反映回該函式外部。這是因為僅將指標的副本傳遞給函式。可以說「透過指標傳遞」就是按值傳遞指標。在大多數情況下，這不會出現問題。但當你修改函式內部的指標時，問題就來了。您只是修改指標的副本，而不是修改變數，而原始指標保持不變。
語法
函式語法
傳回型態 函式名(資料型態*& 別名) {
別名 = 其它記憶體位址
}
呼叫函式語法
函式(指標)
程式碼
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int global_var = 100;
// a function with “Reference to pointer” parameter
void changeReferenceValue(int*& ptr_ptr){
ptr_ptr = &global_var;
}
int main() {
int var = 1;
int* pointer_to_var = &var;
cout << "Before:" << *pointer_to_var << endl;
//把指標變數名傳進函式中
changeReferenceValue(pointer_to_var);
cout << "After:" << *pointer_to_var << endl;
return 0;
}
Before:1
After:100
函式參數指標的指標與指標的別名程式差異
指標的別名
int global_var = 100; // a function with “Reference to pointer” parameter void changeReferenceValue(int*& ptr_ptr){ ptr_ptr = &global_var; } int main() { int var = 1; int* pointer_to_var = &var; cout << "Before:" << *pointer_to_var << endl; //把指標變數名傳進函式中 changeReferenceValue(pointer_to_var); cout << "After:" << *pointer_to_var << endl; return 0; }
指標的指標
int global_var = 100; void changePointerValue(int** ptr_ptr){ *ptr_ptr = &global_var; //改為指向global_var } int main() { int var = 1; int* pointer_to_var = &var; //指向var cout << "Before:" << *pointer_to_var << endl; //passing the address of the pointer //把指標的位址傳進函式中 changePointerValue(&pointer_to_var); cout << "After:" << *pointer_to_var << endl; return 0; }
參考指向指標與new
參考文章
new會返回動態配置記憶體的開始位址，將別名指向new返回的新位址。
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#include <iostream>
using namespace std;
//宣告一個函式initAddress() 別名是ref_to_p，指向外部傳入的指標
void initAddress(int*& ref_to_p){
//印出別名指向的記憶體位址
cout << "Before address = " << ref_to_p << endl;
//動態配置記憶體位址，位址存放的內容為10，使用new會返回動態配置記憶體的開始位址。
ref_to_p = new int(10);
//印出別名指向的記憶體位址
cout << "After address= " << ref_to_p << ",After value = " << *ref_to_p << endl;
}
int main() {
//宣告指標p，初始化為nullptr，也就是沒有指向任何位址
int* p = nullptr;
//呼叫函式initAddress，引數為指標p
initAddress(p);
//印出指標p的位址，印出指標p指向的位址，對指向的位址取出內容。
cout << "== outside == " << endl;
cout << "outside pointer address = " << p << "，outside pointer value = " << *p << endl;
return 0;
}
Before address = 0x0
After address= 0x600000004010,After value = 10
== outside ==
outside pointer address = 0x600000004010，outside pointer value = 10