SV 类继承的多态性问题（NVDIA2019笔试）

1、原题

class class_a;
        virtual function void print_name();
            $display("this is class_a");
        endfunction
endclass

class class_b extends class_a;
        virtual function void print_name();
            $display("this is class_b");
        endfunction
endclass

module test;
    initial begin
    begin:class_test1
            class_a a;
            class_b b;
            a = new();
            b = new();
            a.print_name();
            b.print_name();
    end
    begin:class_test2
            class_a a;
            class_b b;
            b = new;
            a = b;
            a.print_name();
            b.print_name();
    end
    begin:class_test3
            class_a a;
            class_a a2;
            class_b b;
            class_b b2;
            b = new();
            a = b;
            a2 = new();
            b2 = b;
            a2.print_name();
            $cast(a2,a);
            a2.print_name();
            b2.print_name();
    end
    end

endmodule

2、执行结果

　　1）使用a=new会创建一个新对象，并且将句柄a指向这个新创建的对象；使用a=b不会创建新对象，只是将句柄a指向已经存在的对象b

　　2）原因：基类中display()为虚函数，具体调用哪个x.dispaly()取决于句柄x所指向的对象类型，而不是取决于x这个句柄本身的类型

# this is class_a
# this is class_b
# this is class_b
# this is class_b
# this is class_a
# this is class_b
# this is class_b

　　3）原因：同2

# this is class_a
# this is class_b
# this is class_b
# this is class_b
# this is class_a
# this is class_b
# this is class_b

　　4）原因：基类中display()不为虚函数，具体调用哪个x.dispaly()取决于x这个句柄本身的类型

# this is class_a
# this is class_b
# this is class_a
# this is class_b
# this is class_a
# this is class_a
# this is class_b