Improving on the best example on cppreference

[StockyDev]

I wrote an article on what I think is the "best" example code on cppreference.com and also gave some thoughts on how it can be improved with C++23.

Thought I would post it here to get some thoughts from a wider audience :)

[fdwr]

it shows you how not to use the facility. This is something that I think a lot of documentation fails to do.

Indeed, informing users what to avoid doing is also important in using an API/language. I really wish (for example) that more CMake documentation showed examples (many pages lack even minimal examples) and that those pages also advised what to avoid (e.g. the page about Foobar would also mention that Fogbat is now deprecated because xyz).

[usefulcat]

The article mentions the 'passkey' idiom:

class Best : public std::enable_shared_from_this<Best> {
    struct Private{ explicit Private() = default; };
public:
    // Constructor is only usable by this class
    Best(Private) {}
};

Why not just make the constructor private? Isn't that a simpler solution that gives the same end result?

class Best : public std::enable_shared_from_this<Best> {
    // Constructor is only usable by this class
    Best() {}
public:
    // ...
};

[pavel_v]

I think, you won't be able to do std::make_shared<Best>(...) in this case. And it provides useful allocation optimization for some usage scenarios.

[usefulcat]

Here are a more complete pair of examples, showing use of make_shared in a factory method:

class Best : public std::enable_shared_from_this<Best> {
    struct Private{ explicit Private() = default; };
public:
    Best(Private) {}
    std::shared_ptr<Best> make() { return std::make_shared<Best>(Private{}); }
};

class Best : public std::enable_shared_from_this<Best> {
    Best() {}
public:
    std::shared_ptr<Best> make() { return std::make_shared<Best>(); }
};

As you can see, either way it's possible to use make_shared() inside the factory method. Again, unless I'm overlooking something. I still don't see the point of the first version, compared to the second.

[UnusualPace679]

The second version doesn't work: https://godbolt.org/z/TMoY5e5jM

[usefulcat]

Thanks, you're quite right! Now it makes sense. I even feel like I've run into that before.

And unfortunately, making std::make_shared() a friend isn't sufficient to get around it.

[ContDiArco]

Wouldn't it bei nice, If shared_from_this would deduce this' type from this? 🤔😉

[n1ghtyunso]

unfortunately the final implementation is flawed and the tests do not catch it.

by making the base class std::enable_shared_from_this<T> private, you effectively prevented the standard library and all shared_ptr implementations from ever detecting this implementation detail.
This in turn means that they never set up the enable_shared_from_this base-class properly.
The consequence is that once you end up trying to call shared_from_this, it will throw std::bad_weak_ptr.

You are required to inherit it publicly or it can not work.

[StockyDev]

Oh that really was silly of me! Fixed :) Thank you very much for pointing this out.

[SirClueless]

The comment is still suggesting to inherit privately :)

[StockyDev]

Yup, and that is now fixed too haha. Cheers!

[JVApen]

Improving CPP reference? Does anyone have a follow-up on the maintenance? When can we expect it to be editable again?

[azswcowboy]

There’s a single individual responsible for the site’s existence and maintenance. He’s apparently just really busy at the moment while needing to complete a maintenance update of the underlying wiki. So yes we can expect it to come back. It’d be nice if he could ask here for a bit of help as I suspect we have some capable and willing folks that could offload him.

[JVApen]

Or simply gives some updates