const的用法

过一段时间不看，就记不清楚了。在此记录一下

int a1=3;   ///non-const data
const int a2=a1;    ///const data
int * a3 = &a1;   ///non-const data,non-const pointer
const int * a4 = &a1;   ///const data,non-const pointer
int * const a5 = &a1;   ///non-const data,const pointer
int const * const a6 = &a1;   ///const data,const pointer
const int * const a7 = &a1;   ///const data,const pointer