C Language Pointer usage details (iii) Double pointer

Two-weight pointer Example 1:

• Question

    int **ptr1 = NULL;    cout<< "Situation one   ptr1 = =" <<ptr1<<endl;    cout<< "Situation one  *ptr1 = =" <<*ptr1<<endl;    cout<< "situation one **ptr1 = =" <<**ptr1<<endl;   Output: 0  illegal illegal

• Reason:

1) Three-province pointers: The type of pointer ptr1 is int * *, to the type int*, specified as empty 2) *PTR1 is the point that ptr points to, that is, the null point, illegal 3) **PTR1 is the pointing point that ptr points to, that is, pointing to an empty point, illegal Example 2:

• Question

    int * P1 = (int *) malloc (sizeof (int));    int **ptr2 = &p1;    cout<< "Situation two   ptr2 = =" <<ptr2<<endl;    cout<< "Situation two  *ptr2 = =" <<*ptr2<<endl;    cout<< "situation two **ptr2 = =" <<**ptr2<<endl;

• Practice:

#include <bits/stdc++.h>using namespace Std;int main () {    int * p1 = (int *) malloc (sizeof (int));    int **ptr2 = &p1;    cout<< "Situation two     P1 = =" <<p1<<endl;    cout<< "Situation two    *p1 = =" <<*p1<<endl;    cout<< "Situation two    &p1 = =" <<&p1<<endl;    cout<< "Situation two   ptr2 = =" <<ptr2<<endl;    cout<< "Situation two  *ptr2 = =" <<*ptr2<<endl;    cout<< "situation two **ptr2 = =" <<**PTR2<<ENDL;}

　

• Reason:

1) Three-province pointer: the type of pointer p is int *, which points to the type int, initialized to not specify its point 2) Three-province pointers: The type of pointer ptr2 is int * *, the point of type int*, the address of the pointer P (note that at this point ptr2 and &p type are int**, you can assign values) 3) *PTR2 is the ptr2 point, that is, &p1 point, for the P1 4) **PTR2 is the point of the ptr2 point, that is, *ptr2 Point, for *p1 5) The pointer is the address, so as &p1 points to P1 Example 3:

• Question

   int * P2 = (int *) malloc (sizeof (int));    int num = 6;    P2 = #    int **PTR3 = &p2;

• Practice:

#include <bits/stdc++.h>using namespace Std;int main () {    int * p2 = (int *) malloc (sizeof (int));    int num = 6;    cout<< "situation three     P2 = =" <<p2<<endl;    P2 = #    int **PTR3 = &p2;    cout<< "situation three     P2 = =" <<p2<<endl;    cout<< "situation three    *p2 = =" <<*p2<<endl;    cout<< "situation three    &p2 = =" <<&p2<<endl;    cout<< "situation three   &num = =" <<&num<<endl;    cout<< "situation three   PTR3 = =" <<ptr3<<endl;    cout<< "situation three  *PTR3 = =" <<*ptr3<<endl;    

　　

• Reason:

1) Three-province pointers: The type of the pointer p2 is int *, the type of int, the initialization is not pointed, and the pointer is &num (note that at this point P2 and &num type are int*, you can assign a value) 2) Three-province pointers: The type of pointer ptr3 is int * *, the point of type int*, the address of the pointer P2 (note that at this point ptr2 and &p2 type are int**, you can assign a value) 3) *PTR3 is the PTR3 point, that is, &p2 point, for the P2 4) **PTR3 is the point of the Ptr3 point, that is, *ptr3 Point, for *p2 5) The pointer is the address, so as &num points to num, as &p2 points to P2Spit Spit Spit Spit Spit : Write blog is tired ah ah ah ah, blog Park Why so bad use, is I open the way wrong????

C Language Pointer usage details (iii) Double pointer