只读指针变量和只读变量指针

int a = 100;
int b = 200;
//一个const指针指向int型变量
int *const p = &a;
//ERROR 指针变量不可修改
//p = &b;
//值可以修改
*p = 150;

int a = 100;
int b = 200;
//一个指针指向一个const整型变量
int const *p1 = &a
//指针变量可以修改
p2 = &b;
//ERROR 值不可修改
//*p2 = 300;

int a = 100;
int b = 200;
//一个const指针指向一个const整型变量
int const *const pp = &a ;
//ERROR 指针变量不可修改
//pp = &b;
//ERROR 值也不可修改
//*pp = 1000;