In which order should classes be declared in C++?

Say I got this C++ code:

class class1{
    class2 *x;
}

class class2{
    class1 *x;
}

The compiler would give an error in line 2 because it couldn't find class2, and the same if i switched the order of the classes. How do I solve this?

From stackoverflow

• Declare class2 first:

  class class2;
  class class1{
      class2 *x;
  };
  
  class class2{
      class1 *x;
  };
  

• Two things - one, you need semicolons after class declarations:

  class class1{
      class2 *x;
  };
  
  class class2{
      class1 *x;
  };
  

  Two, you can create a declaration in front of the definitions of the classes. That tells the compiler that this class exists, and you have yet to define it. In this case, put a class2 declaration in front of the definition of class1:

  class class2 ;
  
  class class1{
      class2 *x;
  };
  
  class class2{
      class1 *x;
  };
  

• Refer Forward declarations