[error] invalid use of incomplete type uses an undefined type

Today, when writing about Altman's fight against the monster , Find a strange problem , I have defined two base classes Ultraman and Monster, One Monster Subclasses of Boss, Then the two base classes have collusion with each other , They all use each other's type to define themselves more or less , Then I declare another class before the first class implementation ：

After compiling, an error is reported ：

Then it says you can't use incomplete class types ：

I began to feel confused , Clearly my two classes are well defined , How can I say that I have no definition . I also stated above .

Then after two hours of searching by me and another senior in junior year , All kinds of investigation , Finally realized a problem ：

Because these two classes are colluding with each other , So when one class uses another class for object instantiation , Another class will also find this class object to instantiate , And they have not been defined yet , In a nutshell , I need you to do something for me A, But you want to do something A Need me to do something B, And I do things B It must be based on you doing things for me A Under the premise of .

So I found a solution ：

Class definition and class implementation are separated .

Class implementation is written after two class definitions .

done .