[distinguish the meaning and usage of constant pointer and pointer constants const int * and int * const]

Want to find out this matter , First of all, the key is to understand ：

const Belongs to modifier , The key is to see const The position of the decoration is there

1. Constant pointer

 The so-called constant pointer , Constant modifier Const At the front ： Such as Const int *a

here const Modification is int, and int Defines an integer value
therefore a The object that is pointed to value Cannot pass a To modify the , however You can give a To assign a value , Make it point to different objects
eg:
const int *a ;
const int b = 1;
int c = 1;
a = &b //ok additional ： Note that you can't pass a To modify the b value
a = &c //ok additional ： although c Itself is not a constant
*a = 2 //error That's the topic , Can not be modified through a The object value pointed to , Finally, give value The object is c**, So it can't pass a To modify the c value .

2. constant pointer

 The so-called pointer constant , Is that the pointer is at the front   Such as ：Int * const a

here const Modification is a ,a It represents a Pointer address
Therefore, it cannot be assigned to a Other address values , But it can be modified a The value of the point
It's a little different cont int *a The opposite means , No more examples

eg:
Int * const a ;
const int b = 1;
int c = 1;
a = &b //OK First definition , The pointer can point to any variable
a = &c //error a The pointer has pointed to b Variable , So you can't point to other
*a = 2 //OK This means adopt The pointer a To modify the b Variable Value , therefore b Variable from previous 1 become 2