Can someone help me solve this error?

Question

https://code.sololearn.com/caE3eP269gvt/?ref=app
https://code.sololearn.com/caE3eP269gvt/?ref=app
https://code.sololearn.com/caE3eP269gvt/?ref=app

Answer 1

the error you are getting is the result of using a forward declaration and then trying to call a method or access a member without providing the full declaration.( forward declaration only says that a particular class will be defined later )
so as far as the compiler is concerned you can't use any of them until "XYZ" is fully declared.

https://code.sololearn.com/ck51cd3dLwgJ/#cpp

Answer 2

thank you so much