C++ : How to avoid ''identifier' uses undefined class/struct/union 'name'' error when forward declar

C++ : How to avoid "'identifier' uses undefined class/struct/union 'name'" error when forward declaration is not enough?
To Access My Live Chat Page,
On Google, Search for "hows tech developer connect"

As I promised, I have a secret feature to share with you.
This is a YouTube's feature which works on Desktop.
First, Make sure this video is playing.
After that, type the word 'awesome' on your keyboard.
The progress bar on YouTube will be altered to a flashing rainbow.

Here's a brief description of who I am,
Hello there, my name is Delphi.
I help you answer your questions.
C++ : How to avoid "'identifier' uses undefined class/struct/union 'name'" error when forward declaration is not enough?
Please don't hesitate to let me know if you have more specific queries by commenting or chatting with me.
Your input is valuable, so please comment below if you have an answer or any insights to the answer.
Your answer will be recognized and appreciated, and I will 'heart' it to show my appreciation.
is C++ when uses class/struct/union "'identifier' forward How declaration error avoid : undefined 'name'" enough? to not