Secondary pointer

1. Second level pointer basic concept （ Data type of pointer variable address ）

Let's take a moment to look at an example , Open the prelude to this upcoming . Consider the following statements ：

int a = 12; int *b = &a;

They allocate memory as shown in the figure below ：

 

Suppose we have a second 3 A variable , named c, And initialize it with the following statement ：

c = &b;

Its general appearance in memory is as follows ：

 

The problem is ：c What is the type of ？ Obviously it's a pointer , But what does it point to ？ Variable b It's a “ Pointer to an integer ”, So anything that points to b The type of must be pointing to “ Pointer to an integer ” The pointer to , More colloquially , Is a pointer to a pointer .

Is it legal ？ Yes ！ Pointer variables are the same as other variables , Occupy a specific location in memory , So use & It is legal for the operator to get its address .

So how is this variable declared ？

int **c = &b;

So this **c How to understand that ？* The operator has the combination of thinking from the right , So this expression is equivalent to *(*c), We apply for jobs layer by layer from the inside out .*c visit c The point is , We know it's a variable b. The second indirect access operator accesses the address that this location points to , That's the variable a. The pointer of a pointer is not hard to understand , Just watch out for all the arrows , If the indirect access operator appears in the expression , You're going to follow the arrow to where it's pointing .

 

2.  The second level pointer is used as the formal parameter output feature

Explain :a[2] and *a+2 Same

The output characteristic of the secondary pointer as a parameter refers to the allocation of memory by the called function , The main function uses .

// Modulated function , By the parameter n Determine how many elements of memory are allocated void allocate_space(int **arr,int n){ // Heap allocation n individual int Type element memory int *temp = (int *)malloc(sizeof(int)* n); if (NULL == temp){ return; } // Initialize the memory int *pTemp = temp; for (int i = 0; i < n;i ++){ //temp[i] = i + 100; *pTemp = i + 100; pTemp++; } // Pointer indirect assignment *arr = temp; } // Print array void print_array(int *arr,int n){ for (int i = 0; i < n;i ++){ printf("%d ",arr[i]); } printf("\n"); } // Secondary pointer output characteristics ( Allocate memory by the called function ) The main function void test(){ int *arr = NULL; int n = 10; // to arr Pointer indirect assignment allocate_space(&arr,n); // Output arr Points to the memory of the array print_array(arr, n); // Release arr The value of the memory space pointed to if (arr != NULL){ free(arr); arr = NULL; } }

3. The secondary pointer is used as a formal parameter input feature

The second level pointer is used as formal parameter input, which means that the main calling function allocates memory , The called function uses it

// Print array void print_array(int **arr,int n){ for (int i = 0; i < n;i ++){ printf("%d ",*(arr[i])); } printf("\n"); } // Secondary pointer input characteristics ( Allocate memory by the calling function ) void test(){ The stack area applies for data memory int a1 = 10; int a2 = 20; int a3 = 30; int a4 = 40; int a5 = 50; int n = 5; int** arr = (int **)malloc(sizeof(int *) * n); arr[0] = &a1; arr[1] = &a2; arr[2] = &a3; arr[3] = &a4; arr[4] = &a5; Print heap data print_array(arr,n); Release the heap area free(arr); arr = NULL; }