指向const的指针：

形如

int a = 5;

int b = 6;

const int *ptr = &a;

*ptr = 8; //error

ptr = &b; //ok;

const 指针 ：

形如

int a = 5;

int b = 6;

int * const ptr = &a;

*ptr = 8; //ok

ptr = &b; //error