int a1=3; //可随意修改
const int a2=a1; //a2不能被修改，如a2 = 3
const int a2=a1; //a2不能被修改，如a2 = 3
int * a3 = &a1; //a3和*a3都可以修改
const int * a4 = &a1; //a4可以修改，*a4不可以修改
int * const a5 = &a1; ///*a5可以修改，a5不可以修改
int const * const a6 = &a1; //a6和*a6都不可以修改
const int * const a7 = &a1; //a7和*a7都不可以修改
const int * a4 = &a1; //a4可以修改，*a4不可以修改
int * const a5 = &a1; ///*a5可以修改，a5不可以修改
int const * const a6 = &a1; //a6和*a6都不可以修改
const int * const a7 = &a1; //a7和*a7都不可以修改
