class A{public:unsigned a[2][2];inline A(){memset(a,0,sizeof a);}
    inline unsigned *const operator[](const unsigned i){return a[i];}
    inline A operator*(const A b){A c;
        c[0][0]=a[0][0]*b[0][0]+a[0][1]*b[1][0],
        c[0][1]=a[0][0]*b[0][1]+a[0][1]*b[1][1],
        c[1][0]=a[1][0]*b[0][0]+a[1][1]*b[1][0],
        c[1][1]=a[1][0]*b[0][1]+a[1][1]*b[1][1];return c;
	}
};

报错信息：[Error] passing 'const A' as 'this' argument of 'unsigned int* A::operator[](unsigned int)' discards qualifiers [-fpermissive]