2. Const pointer

const Qualifiers are used to mark a variable as read-only , But it's a little complicated when decorating pointers .

First ,const int *pa and int const *pa The effect is the same , These two are completely equivalent , This const Modification is *pa This whole , in other words *pa This whole is read-only , however pa（ No addition *） It can be modified . The variable it points to can also be modified . for instance

int a = 100;
const int *pa = &a;
printf("%d\n",*pa);//100
a = 200;
printf("%d\n",*pa);//200

int a = 100;
int b = 200;
const int *pa = &a;
printf("%d\n",*pa);//100
pa = &b;
printf("%d\n",*pa);//200

int a = 100;
const int *pa = &a;
printf("%d\n",*pa);//100
*pa = 200;// Report errors ,*pa yes const Type , read-only 

and int * const pa in pa It's also a pointer , But this const Modification is pa Instead of *pa This whole , in other words , At this time, you can use the *pa To modify the a The value of the , however pa It's immutable , That is to say pa Can only point to a 了 , You can't point him at b, because pa The value of is the address of the target it points to , This is read-only .

Of course we can use const int * const pa This format , In this way, it is pa and *pa All read-only .