[fjsolwmv]

Can you give in an example?

Keeping a debug reference at end of transaction (via a shared-reference type, since a non-shared RAII reference type could never get into that state) isn't a coding error, it's a design error -- the development intentionally requested contradictory things. That is solved by using weak references if you don't want a debug tool to force an object to stay alive.

[rbehrends]

It is not a design error. You are making the mistake of equating reachability of a reference with making it legal to access it. And the problem is that the two often aren't one and the same.

As a simple example, you may still want to access a resource after it has been released. Closing a network connection, for example, does not mean that accessing it is invalid. The connection may still have state (such as statistics collected or whether a non-blocking close was clean) that is perfectly legal to access after release (and in fact may only be consistent/observable afterwards).

The Eiffel FILE class [1], for example, allows you to call `is_closed` at any time (as well as the various `is_open` functions). This is necessary because `not is_closed` is evaluated as a precondition for many other operations.

A more complex example is a resource that is shared by many threads. Whether that resource is valid is often not a question of whether a reference is reachable, but a function of complex distributed state. Sometimes this can be solved by atomic reference counting, but even then atomic reference counting is expensive.

[1] https://archive.eiffel.com/products/base/classes/kernel/file...