[Interest] QExplictlySharedDataPointer vs QSharedPointer

[Elvis Stansvik]

Hi all,

QSharedData and QImplicitlySharedDataPointer (QISDP) is a common
combination used to implement implicit sharing, both in Qt itself and
in Qt-using projects.

QExplicitlySharedDataPointer (QESDP) is a cousin of QISDP with the
important difference that you have to explicitly call detach() to
detach from the underlying data.

QSharedPointer is Qt's own "smart pointer" to shared data, similar to
std::shared_ptr.

I'm curious about the role/use for QESDP compared to QSharedPointer.
The main differences I can see is

 * QESDP requires that the pointed-at object derive from QSharedData.
 * QSharedPointer provide no easy means to detach/make copies of the
underlying object

But regarding this second difference, wouldn't it be easy enough to
just take the raw pointer to the underlying object with data(), and
make a copy of that?

In short, I fail to see a strong use case for QESDP compared to using
a QSharedPointer. I'm probably missing something.

Does anyone know of a code base that makes liberal use of QESDP, or
could you share how you're making use of it? It's easy to find uses
for the implicit variant (QISDP).

In general I'm curious to hear advise about the do's and don'ts when
it comes to QISDP, QESDP and QSharedPointer when designing the
innermost parts (data model) of a Qt application. I've heard some
warning words about using QISDP too library, because of the risks of
"accidental detach".

Thanks in advance,
Elvis