C++ 指針 * 和 引用 &

指針

先看一個簡單的例子：

#include <iostream>
using namespace std;

int main() {
    // your code goes here
    int num = 123;
    int* p = &num;
    cout<<"p:"<<p<<endl;
    cout<<"*p:"<<*p<<endl;
    cout<<"num:"<<num<<endl;
    cout<<"&num:"<<&num<<endl;
    return 0;
}

運行結果：

p:0x7ffc2861549c
*p:123
num:123
&num:0x7ffc2861549c

p是指向num地址的指針，所以p的值為num的地址。可以給*p賦值，此時num值也會發生相應的變化，但是不會因此而改變p所指向的地址。

#include <iostream>
using namespace std;

int main() {
    int num = 123;
    int* p = &num; // 這里&為取址
    cout<<"*p = "<<*p<<endl;
    cout<<"p = "<<p<<endl;
    
    *p = NULL;
    cout<<"*p = "<<*p<<endl;
    cout<<"p = "<<p<<endl;
    cout<<"num = "<<num<<endl;
}

*p = 123
p = 0x7ffccb7a153c
*p = 0
p = 0x7ffccb7a153c
num = 0

引用

C++引用

類型標識符 &引用名=目標變量名

#include <iostream>
using namespace std;

int main() {
    int num1 = 1, num2 = 2;
    int &ref1 = num1, &ref2 = num2; ///引用必須要初始化
    cout<<"num1 = "<<num1<<",num2 = "<<num2<<endl;
    cout<<"ref1 = "<<ref1<<",ref2 = "<<ref2<<endl;
    ///修改引用的值將改變其所綁定的變量的值
    ref1 = -1;
    cout<<"num1 = "<<num1<<",ref1 = "<<ref1<<endl;
    
    ///將引用b賦值給引用a將改變引用a所綁定的變量的值，
    ///引用一但初始化(綁定)，將始終綁定到同一個特定對象上，無法綁定到另一個對象上
    ref1 = ref2;
    cout<<"num1 = "<<num1<<",ref1 = "<<ref1<<endl;
    return 0;
}

num1 = 1,num2 = 2
ref1 = 1,ref2 = 2
num1 = -1,ref1 = -1
num1 = 2,ref1 = 2

指向指針的指針

#include <iostream>
using namespace std;

int main() {
    int val = 1;
    int *p1 = &val;
    int **p2 = &p1;///**聲明一個指向指針的指針
    cout<<"val = "<<val<<endl;
    cout<<"p1 = "<<p1<<", *p1 = "<<*p1<<endl;
    cout<<"p2 = "<<p2<<", *p2 = "<<*p2<<",**p2 = "<<**p2<<endl;
    return 0;
}

val = 1
p1 = 0x7ffe520c3d34, *p1 = 1
p2 = 0x7ffe520c3d38, *p2 = 0x7ffe520c3d34,**p2 = 1

指針與數組

#include <iostream>
using namespace std;

int main() {
    int arr[5][5];
    arr[2][1] = 666;
    cout<<*(*(arr + 2) + 1)<<endl;
    return 0;
}

#include <iostream>
using namespace std;

int main() {
    int arr[100];
    for (int i = 0; i < 100; i++)
        arr[i] = i;
    int *p = arr;  //等價於int *p = &arr[0];
    //數組的變量名就是一個指針
    cout<<"p = "<<p<<",*p = "<<*p<<endl;  //p = 0x7ffe3b44b8b0,*p = 0
    
    int t = 100;
    while (t--)
        ///可以直接對指針進行加減運算，就和迭代器一樣
        cout<<*(p++)<<endl;  //輸出0~99
        
    ///指針可以做差：
    int *p2 = &arr[10], *p3 = &arr[20];
    cout<<"p2 - p3 = "<<(p2 - p3)<<endl;  //p2 - p3 = -10
    
    ///還可以比比較大小：
    cout<<(p2 < p3 ? p3 - p2 : p2 - p3)<<endl; //10
    return 0;
}