Pointer to parent class cannot point to child class object

The pointer to the parent class points to the object of the subclass, which is called polymorphic in C + +.

Of course, polymorphism can also have a deeper explanation.

But why do the pointers to the parent class sometimes point to objects that are not subclasses?

There are two files in the following code:

B.hclass b:public a{};//main.cpp Class A; Class B;  void Fun (b* b) {A * A = B; There's a mistake here}

The reason is this, and sometimes we want to improve the speed of compiling,

Will do some pre-declaration, such as Class A tells compiler A to exist.

Then we can a; Define an object

But when you're going to write this, a * = b;

The compiler does not know that a is the parent class of B, so

It's an error. The simplest way is to include the header file of B.

Pointer to parent class cannot point to child class object