Virtual Equal Function on C++ - c++

Question

I am trying to do something like:
class A
{
public:
A() = default;
~A() = default;
public:
bool operator==(const A& a)
{
return this->equal(a);
};
private:
virtual bool equal(const A& other) const = 0;
};
class B : public A
{
public:
B() = default;
~B() = default;
private:
virtual bool equal(const A& other) const override
{
const B* b = dynamic_cast<const B*>(&other);
if (!b)
return false;
return m_example == b->m_example;
}
private:
double m_example; //This is just an example data I don't need specific solution for double :)
};
Goal: Only equality possible is between classes of same type = B.
Is that ok?
Is there a standard solution to have a virtual equal operator?
I will be inherting from A a lot (4/8 classes), how can I do this in a clean way, the check if the pointer is null looks very ugly.
Could you please help me?

Answer 1

It's a matter of opinion whether dynamic casts are ugly, or not. It's an endless debate, but it doesn't matter here. That's because dynamic casts are not needed here. This can be done, cleanly, using inheritance:
class B; // Forward declaration.
class A {
// Same as above
private:
virtual bool equal(const A& other) const = 0;
virtual bool equal(const B& other) const { return false; }
};
// ... B class:
bool equal(const A& other) const override
{
return other.equal(*this);
}
bool equal(const B &other) const override
{
return m_example == other.m_example;
}