const理解
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
const用来修饰类型,const对其左边所有的类型起作用,对右边最少的类型起作用