How can I know if an object is in use or not?

Garbage collection normally isn't done in C++. Code is expected to free objects explicitly (e.g., by using delete) or implicitly (when a unique_ptr goes out of scope).

However, here's an article[^] about a garbage collector that I wrote. Objects are created in blocks allocated from pools, and a background garbage collector recovers objects that aren't "claimed" at the application level. This is basically a mark and sweep strategy, but applications must claim in-use objects because the system doesn't manage pointers and therefore can't tell which objects have owners. Applications are still expected to free objects explicitly, so the background garbage collector serves as a backup to recover from memory leaks.

For that you need to understand the concept of scope which means life time of an object. Here is some explanation of the scope in C++.

You must also understand the differences between heap and stack memory in C++.

You should read both sites carefully and understand them for your task or you will fail. ;-)