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Pointer&Reference

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Cpp Operation Note

不同於Python及Java, c++同時支援Pointer及Reference。

名詞解釋

  1. Value (值): 記憶體內儲存的變數
  2. Pointer (指標): 存儲變數之記憶體的絕對位置
    *Pointer會另開記憶體存取地址。
  3. Reference (引用): 調出變數所存在記憶體之絕對位置
    *Reference導向所附記憶體位置，不另開記憶體。
  4. 操作符 (&): 取變數所在之記憶體絕對位置。
  5. 操作符 (*): 對記憶體位置取出值。

初始化

  1. Pointer初始化:
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int a = 5;
int *ptr = &a;

//or

int *ptr;
ptr = &a;

//ptr = 0x------------;
//*ptr = 5;
  1. Reference初始化:
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int a = 5;
int &ref = a;

//but

int &ref;
ref = a; // This operation is incorrect.

int *ptr = &a;

//ref = *ptr = 5;
//&ref = ptr = 0x------------;
  1. 重新附值:
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int a = 3;
int b = 5;
int *ptr;

ptr = &a;
ptr = &b;

Pointer可以重新附值

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int a = 3;
int b = 5;

int &ref = a;
int &ref = b;//This wil throw an error of "multiple declaration is not allowed"

Reference不可被重新附值及宣告

實裝例swap

1. int 

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#include <bits/stdc++.h>
 using  namespace std;

 void swap_int(int a, int b){
    swap(a, b);
    cout << "In function:" << endl;
    cout << " a = " << endl;
    cout << " b = " << endl;
    }
 int main(){
    int a = 1 ;
    int b = 2 ;
    swap_int(a, b);
    cout << "In main:" << endl;
    cout << " a = " << a << endl;
    cout << " b = " << b << endl;
    return  0 ;
}

output

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in function:
 a = 2
 b = 1
in main:
 a = 1
 b = 2

交換失敗，原因是a,b在main及function中為兩獨立區域變數，不互相干擾。

2. *int 

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#include <bits/stdc++.h>
using  namespace std;

void swap_value_ptr(int *ptra, int *ptrb){
    swap(*ptra, *ptrb);
}

int main(){
    int a = 1 ;
    int b = 2 ;
    cout << "Before swap:" << endl;
    cout << " a = " << a << endl;
    cout << " b = " << b << endl;
    cout << " &a = " << &a << endl;
    cout << " &b = " << &b << endl;
    swap_value_ptr(&a, &b);
    cout << "After swap:" << endl;
    cout << " a = " << a << endl;
    cout << " b = " << b << endl;
    cout << " &a = " << &a << endl;
    cout << " &b = " << &b << endl;
    return  0 ;
}

將a,b記憶體位置傳入function，並交換兩記憶體內value

output

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Before swap:
 a = 1
 b = 2
 &a = 0x7ffcdb13f900
 &b = 0x7ffcdb13f904
After swap:
 a = 2
 b = 1
 &a = 0x7ffcdb13f900
 &b = 0x7ffcdb13f904

從output可以看出a,b變數之記憶體維持而變數value交換。

3. &int 

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#include <bits/stdc++.h>
using  namespace std;

void swap_ref(int &refa, int &refb){
    swap(refa, refb);
}

int main(){
    int a = 1 ;
    int b = 2 ;
    cout << "Before swap:" << endl;
    cout << " a = " << a << endl;
    cout << " b = " << b << endl;
    cout << " &a = " << &a << endl;
    cout << " &b = " << &b << endl;
    swap_ref(a, b);
    cout << "After swap:" << endl;
    cout << " a = " << a << endl;
    cout << " b = " << b << endl;
    cout << " &a = " << &a << endl;
    cout << " &b = " << &b << endl;
    return  0 ;
}

原理同上一個，改用引用方式調換value

output

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Before swap:
 a = 1
 b = 2
 &a = 0x7ffd068869c0
 &b = 0x7ffd068869c4
1
After swap:
 a = 2
 b = 1
 &a = 0x7ffd068869c0
 &b = 0x7ffd068869c4

從output可以看出a,b變數之記憶體維持而變數value交換。

參考資料： [C++基礎]019_指針和引用（int、int&、int&、int&*、int**）, Pointers vs References in C++