Question

Had a complicated program that kept throwing up the missing default constructor error, and after much tinkering, i found the exact same scenario that gives the same error. What’s wrong with this?

class B;

class A
{
public:
    A() {instance = new B;}
    virtual ~A() {delete instance;}
private:
    A*instance;
};

class B : public A
{
public:
    B(){}
}

can’t forward declare a derived class to be used within the base class?

Answer 1

How can new B succeed if the compiler doesn’t know anything about class B yet? If you move the member function implementations out of class A below the definition of class B, it should work:

class A
{
public:
    A();
    virtual ~A();
private:
    A * instance;
};

class B : public A
{
public:
    B(){}
};

A::A()
{
    instance = new B;
}

A::~A()
{
    delete instance;
}

What exactly are A and B, anyway? Having a base class instantiate a derived class sure is a bit unusual.