How to change this code

bossskinger

#include <iostream.h>
class Base {
public: virtual void fun () = 0;
};
class Test: public Base {
public: virtual void fun () {cout << "test.fun =" << endl;}
};
void main () {
Base a;
Test * p; p =&a;
}

There is an error in this code, but I don't know how to change it, please brothers to help change it

40668477

class Base {
public: virtual void fun () = 0;
};

Base a; // Virtual base class cannot be instantiated

firegun

Base is an abstract class and cannot be instantiated, so Base a; will definitely go wrong and can be changed
Test p;
Base * a =&p;

finefineboy

Test test;
Base * p =&test;
p-> fun ();

plato169

I said in detail:
A class containing pure virtual functions is a virtual base class and cannot be instantiated, so the Base a in your main function is wrong.It is also worth noting that pure virtual functions cannot be fixed in the base class. The body of the function must be overridden in the derived class.
You should use this program to verify the polymorphism of C ++ runtime, first define a pointer to the base class, and then point to the derived class to implement polymorphism.
Test a;
Base * p =&a;
p-> fun ();

firegun

Alas, I don't know why everyone likes to use the term virtual base class, but something like
class Base {
public: virtual void fun () = 0;
};
Such a class containing pure virtual functions should be called an abstract class instead of a virtual base class. The so-called virtual base class, such as the following
class A {....};
class B: virtual public A {....};
class C: virtual public A {....};
Here, A is called the virtual base class of B and C, because B and C use "virtual inheritance" when inheriting A. This inheritance method is used to prevent "diamond multiple inheritance" when multiple inheritance is used. such as:
class D: public B, pubilc C {....};
At this time, "virtual inheritance" can ensure that D saves only one member variable of A.
Ha ha, just be serious about the issues discussed, do n’t blame everyone

sevenbox

Learn

fuleia

I learned a lot, thank you guys ~

天魔狂舞

THANK YOU.