'CLASS' does not name a type

[plopes21]

I have this example below. How to solve this problem?

class ABC{
private:
    XYZ *xyz;   //XYZ is not defined yet
};
 
class XYZ{
private:
    ABC *abc;   //ABC is defined above
};

class ABC{
private:
    XYZ *xyz;   //XYZ is not defined yet
};

class XYZ{
private:
    ABC *abc;   //ABC is defined above
};

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