Optional virutal function in extended class, test if it exists at runtime - c++

Question

class A {
public:
void (*_start)() = 0;
void start() {
cout<<"Le ";
if (_start!=0) _start();
}
A() {}
};
class B : public A {
public:
void _start() {
cout<<"Blop";
}
B(){}
};
class C : public A {
public:
C(){}
};
int main () {//Desired results:
B().start(); //Le Blop
C().start(); //Le
return 0;
}
The following code above does not run, but in the main method I have thier desired outputs.
Is it at all possible to have "optional" virtual methods? To be able to easily overload and impliment a function in the child class, and have it as some null value in the parent class so that its methods can identify if the function is defined or not.
What I get from the code above is Le Le, I assume _start in the parent class differs from the _start in the child class, not by value, but identity.
If so, how is this done? I would like to do it in a tidy way, such as, not having to set _start in the constructor.

Answer 1

Was alittle silly,
I could simply define the optional function as virtual void _start() { }; instead of virtual void _start() = 0;. Having the exact same behavior of the desired code above
Resulting in:
class A {
public:
virtual void _start() { };
void start() {
cout<<"Le ";
_start();
}
A() {}
};
class B : public A {
public:
void _start() {
cout<<"Blop";
}
B(){}
};
class C : public A {
public:
C(){}
};
int main () {//Desired results:
B().start(); //Le Blop
C().start(); //Le
return 0;
}

Answer 2

Without virtual methods, you might do:
class B : public A {
public:
B() {
_start = [](){ std::cout<<"Blop\n";};
}
};
Demo
But you don't need inheritance in fact:
int main () {
A a1{[](){ std::cout<<"Blop\n";}};
A a2;
a1.start(); //Le Blop
a2.start(); //Le
}
Demo
void (*_start)() could be replaced by std::function<void()> _start; to allow more functors (as capturing lambda).

Answer 3

The classes B and C don't exist without class A and class A is part of B and C respectively. With this in mind, class A shouldn't worry if it is inherited by other classes.
With this in mind, you can simplify the code. Below you can find an example houw the code could be:
struct A {
void start() {
cout << "Le ";
}
};
struct B : public A {
void start() {
A::start();
cout << "Blop\n";
}
};
struct C : public A {
void start() {
A::start();
}
};
int main () {
B b;
C c;
b.start();
c.start();
return 0;
}