Introduction to const in C Language

We all know that,ConstThe usage is very flexible. It is still tricky for new users. It is recommended that you take a look at these two articles. You can refer to this article for a detailed introduction to const usage in C ++ and a detailed description of const usage for C ++ beginners.

Const is unfamiliar and familiar to many people. it can be said that the problem is very difficult to understand in some places. when I first learned C, I had a vague understanding of it. I seem to understand it a little, but I can't say why. I would like to share with you my understanding of it all the time. If you have any mistakes, please advise.

Const can be said to be a constant declaration keyword, such as const int a = 2; variable a is a constant variable, and the value of a cannot be changed, this change cannot be changed in a certain sense. instead, the value cannot be changed directly through the symbol. but in fact, the value of a may change. example:

 
 

  
  
#include<stdio.h>  
  
  
int main()  
  
  
{  
  
  
const int a=3;  
  
  
int *p=(int*)&a;  
  
  
*p=100;  
  
  
printf("%d",a);  
  
  
return 0;  
  
  
} 
 
 

In this way, the value of a is changed.

Const can also be said to be a read-only keyword, such

 
 

  
  
int integerArray[10]={0};  
  
  
const int* pointer=integerArray; 
 
 

Here pointer is a pointer variable pointing to a constant variable. pointer Points to a variable that may change, but the value of the variable it points to cannot be changed directly through pointer. For example:

 
 

  
  
*(p+1)=2; 
 
 

This is not acceptable. But if there is another array, integerArray1, p = integerArray1 can be. There is also a variable pointing to which it cannot be changed. This change is a meaningful change. the following example:

 
 

  
  
#include<stdio.h>  
  
  
int main()  
  
  
{   
  
  
int array[10]={0};  
  
  
const int *pointer=array;  
  
  
int *pointer1=(int*)pointer;  
  
  
pointer1[0]=1;  
  
  
printf("%d",array[0]);  
  
  
return 0;  
  
  
} 
 
 

The above example shows that the array cannot be changed directly through pointer, but the array may be changed indirectly through pointer;

Now let's talk about the const declaration of a constant pointer. Its declaration is not like the declaration of a pointer to a constant. its declaration form is: int * const pointer ;. the point of a regular pointer in programming cannot be changed. therefore, the constant pointer must assign an initial value to it during declaration.

Otherwise, you cannot assign a value to it directly. However, if you must assign a value to it, it is acceptable. The following example:

 
 

  
  
#include<stdio.h>  
  
  
int main()  
  
  
{  
  
  
int a=2;  
  
  
int* const p;  
  
  
int **p1=(int**)&p;  
  
  
*p1=&a;  
  
  
printf("%d",*p);  
  
  
return 0;  
  
  
} 
 
 

After the above introduction, I believe that it is not difficult to write a statement on the constant pointer to a constant. its declaration form: cont int * const p; it is not used for too much introduction. A constant pointer to a constant is a combination of a pointer to a constant and a constant pointer.