理解一般指针和指向指针的指针

在说指向指针的指针之前，不得不说指向变量的指针。先看如下示例：

1. 指向整型指针

先看如下示例：

1 #include <iostream> using namespace std; int main() { int a = 5; int * p = &a; cout << "a = " << a << endl << "&a = " << &a << endl << "*p = " << *p << endl << "p = " << p << endl << "&p = " << &p << endl; return 0; }

运行结果如下：

我们先看下内存分配图：

由上图可以清楚的知道，输出整形变量a的值是5，指针变量p的值是001BFD18，而*号的作用是取值，*p即取地址001BFD18中存放的值，即5。

2. 指向字符型指针

先看如下示例：

1 #include <iostream>
using namespace std;

int main()
{
    char a[] = "hello";
    char *p = a;

    cout << "p = " << p << endl
         <<"p = " << (void *) p << endl
         << "*p = " << *p << endl;
        
        
    for(int i = 0 ; i < 5; i++)
    {
        cout << "&a[" << i << "] = "<< (void *)&a[i] << endl;
    }
    return 0;
}

 1 #include <iostream>
 2 using namespace std;
 3 
 4 int main()
 5 {
 6     char a[] = "hello";
 7     char *p = a;
 8 
 9     cout << "p = " << p << endl
10          <<"p = " << (void *) p << endl
11          << "*p = " << *p << endl;
12         
13         
14     for(int i = 0 ; i < 5; i++)
15     {
16         cout << "&a[" << i << "] = "<< (void *)&a[i] << endl;
17     }
18     return 0;
19 }

运行结果图如下：

为什么整型指针p输出的是地址，而字符型指针输出的是字符串呢，字符型指针里存放的不是地址吗？

我们先看下内存分配图：

由上图可以看出，其实p中存放的是地址，只是当cout时，如果指针是字符型指针，那么会输出p中地址指向的内存中的内容(这里是h)直到遇到''才结束。所以直接输出p时会输出hello，而将p强制转换为void *时输出的是地址。

3. 指向整型指针的指针

先看如下示例：

1 #include <iostream>
using namespace std;

int main()
{
    int a[5] = {1, 2, 3, 4, 5};
    int *p = a;
    int **point = &p;

    cout << "a = " << a << endl
        << "p = " << p << endl
        << "&p = " << &p << endl
        << "point = " << point << endl
        << "&point = " << &point << endl;

    for (int i = 0; i < 5; i++)
    {
        cout << "&a[" << i << "] = " << &a[i] << endl;
    }
    return 0;
}

 1 #include <iostream>
 2 using namespace std;
 3 
 4 int main()
 5 {
 6     int a[5] = {1, 2, 3, 4, 5};
 7     int *p = a;
 8     int **point = &p;
 9 
10     cout << "a = " << a << endl
11         << "p = " << p << endl
12         << "&p = " << &p << endl
13         << "point = " << point << endl
14         << "&point = " << &point << endl;
15 
16     for (int i = 0; i < 5; i++)
17     {
18         cout << "&a[" << i << "] = " << &a[i] << endl;
19     }
20     return 0;
21 }

运行结果图如下：

我们先看下内存分配图：

从上图可以看出point指针中存放的是p指针的地址，而p指针中存放的是a[0]的地址。所以*point和p是一样的，前者是取point指针中存放的地址(0025F754)中的值，即取地址0025F754中存放的值(0025F760)，而后者就是0025F760，所以两者是等价的。**point和a[0]是等价的，前者可以写成*p，*p是取p中存放的地址(0025F760)中的值，即地址0025F760中存放的值1。由上可以得出*point等于p， **point 等于 a[0]。通过上图可以清晰的对付诸如*point++等问题。

4. 指向整型指针的指针

先看如下示例：

1 #include <iostream>
using namespace std;

int main()
{
    char *a[] = {"Wel", "To", "China"};
    char **p = a;
    for(int i = 0; i < 3; i++)
    {
        for (int j = 0; j < strlen(a[i]) + 1; j++)
        {
            cout << a[i][j] << "	" << (void *)&a[i][j] << endl;
        }
        cout << endl;
    }
    
    for (int i = 0; i < 3; i++)
    {
        cout << "a[" << i << "] = " << (void *) a[i] << endl
             << "&a[" << i << "] = " << &a[i] << endl;
    } 


    cout << "p  = " << p << endl
         << "&p = " << &p << endl;
    return 0;
}

 1 #include <iostream>
 2 using namespace std;
 3 
 4 int main()
 5 {
 6     char *a[] = {"Wel", "To", "China"};
 7     char **p = a;
 8     for(int i = 0; i < 3; i++)
 9     {
10         for (int j = 0; j < strlen(a[i]) + 1; j++)
11         {
12             cout << a[i][j] << "	" << (void *)&a[i][j] << endl;
13         }
14         cout << endl;
15     }
16     
17     for (int i = 0; i < 3; i++)
18     {
19         cout << "a[" << i << "] = " << (void *) a[i] << endl
20              << "&a[" << i << "] = " << &a[i] << endl;
21     } 
22 
23 
24     cout << "p  = " << p << endl
25          << "&p = " << &p << endl;
26     return 0;
27 }

运行结果图如下：

我们先看下内存分配图：

由上图可以看出a[0]中存放着'W'的地址，a[1]中存放着'T'的地址，a[2]中存放着'C'的地址，只是这些地址都是指向字符型的，所以直接cout的会输出字符串，而指针p中存放着a[0]的地址，所以*p等于a[0],都是获得'W'的地址，即00A778CCC，而**p和a[0][0]等价都获得了地址00A778CCC中存放的值W。由上图我们可以看到字符地址相隔1个字节，而指针地址相隔4个字节，这样就便于++运算，获得下一个地址了，列如++p后，p就指向a[1]，p中存放的是a[1]的地址。