段错误

实现一个矩阵乘法，主函数有矩阵类m1,m2,m3，重载 * ，运行结果正确却有段错误，求解 ？？？

#include
#include
using namespace std;

class matrix{
    private:
        int row;
        int column;
        int **mat;
    public:
        matrix(int r=0,int c=0);
        void display();
        ~matrix();
        void import();
        friend matrix operator * (matrix &a,matrix &b);
};

matrix::matrix(int r,int c):row(r),column(c){
    if(row==0 || column==0) mat=NULL;
    else {mat=new int *[row]; for(int i=0;i>mat[i][j];
}
void matrix::display(){
    for(int i=0;i>r1>>c1;
    matrix m1(r1,c1);
    m1.import();
    cin>>r2>>c2;
    matrix m2(r2,c2);
    m2.import();
    matrix m3(r1,c2);
    m3=m1*m2;
    m3.display();
    return 0;
}