unique_ptr to an opaque struct? (C++) [duplicate]

This question already has an answer here:

• Is std::unique_ptr<T> required to know the full definition of T? 4 answers

A library defines an opaque data type:

struct OpaqueStruct;

and the client code has to obtain and release an OpaqueStruct*. I have access to the library source.

Unfortunately, neither shared_ptr nor unique_ptr cannot store that pointer giving an error: invalid application of ‘sizeof’ to incomplete type.

The best thing I can think of is to borrow the finally guard from this post.

How do I use RAII for opaque struct pointers?