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std::shared_ptr's secret constructor

Friday, 24 July 2015

std::shared_ptr has a secret: a constructor that most users don't even know exists, but which is surprisingly useful. It was added during the lead-up to the C++11 standard, so wasn't in the TR1 version of shared_ptr, but it's been shipped with gcc since at least gcc 4.3, and with Visual Studio since Visual Studio 2010, and it's been in Boost since at least v1.35.0.

This constructor doesn't appear in most tutorials on std::shared_ptr. Nicolai Josuttis devotes half a page to this constructor in the second edition of The C++ Standard Library, but Scott Meyers doesn't even mention it in his item on std::shared_ptr in Effective Modern C++.

So: what is this constructor? It's the aliasing constructor.

Aliasing shared_ptrs

What does this secret constructor do for us? It allows us to construct a new shared_ptr instance that shares ownership with another shared_ptr, but which has a different pointer value. I'm not talking about a pointer value that's just been cast from one type to another, I'm talking about a completely different value. You can have a shared_ptr<std::string> and a shared_ptr<double> share ownership, for example.

Of course, only one pointer is ever owned by a given set of shared_ptr objects, and only that pointer will be deleted when the objects are destroyed. Just because you can create a new shared_ptr that holds a different value, you don't suddenly get the second pointer magically freed as well. Only the original pointer value used to create the first shared_ptr will be deleted.

If your new pointer values don't get freed, what use is this constructor? It allows you to pass out shared_ptr objects that refer to subobjects and keep the parent alive.

Sharing subobjects

Suppose you have a class X with a member that is an instance of some class Y:

struct X{
    Y y;
};

Now, suppose you have a dynamically allocated instance of X that you're managing with a shared_ptr<X>, and you want to pass the Y member to a library that takes a shared_ptr<Y>. You could construct a shared_ptr<Y> that refers to the member, with a do-nothing deleter, so the library doesn't actually try and delete the Y object, but what if the library keeps hold of the shared_ptr<Y> and our original shared_ptr<X> goes out of scope?

struct do_nothing_deleter{
    template<typename> void operator()(T*){}
};

void store_for_later(std::shared_ptr<Y>);

void foo(){
    std::shared_ptr<X> px(std::make_shared<X>());
    std::shared_ptr<Y> py(&px->y,do_nothing_deleter());
    store_for_later(py);
} // our X object is destroyed

Our stored shared_ptr<Y> now points midway through a destroyed object, which is rather undesirable. This is where the aliasing constructor comes in: rather than fiddling with deleters, we just say that our shared_ptr<Y> shares ownership with our shared_ptr<X>. Now our shared_ptr<Y> keeps our X object alive, so the pointer it holds is still valid.

void bar(){
    std::shared_ptr<X> px(std::make_shared<X>());
    std::shared_ptr<Y> py(px,&px->y);
    store_for_later(py);
} // our X object is kept alive

The pointer doesn't have to be directly related at all: the only requirement is that the lifetime of the new pointer is at least as long as the lifetime of the shared_ptr objects that reference it. If we had a new class X2 that held a dynamically allocated Y object we could still use the aliasing constructor to get a shared_ptr<Y> that referred to our dynamically-allocated Y object.

struct X2{
    std::unique_ptr<Y> y;
    X2():y(new Y){}
};

void baz(){
    std::shared_ptr<X2> px(std::make_shared<X2>());
    std::shared_ptr<Y> py(px,px->y.get());
    store_for_later(py);
} // our X2 object is kept alive

This could be used for classes that use the pimpl idiom, or trees where you want to be able to pass round pointers to the child nodes, but keep the whole tree alive. Or, you could use it to keep a shared library alive as long as a pointer to a variable stored in that library was being used. If our class X loads the shared library in its constructor and unloads it in the destructor, then we can pass round shared_ptr<Y> objects that share ownership with our shared_ptr<X> object to keep the shared library from being unloaded until all the shared_ptr<Y> objects have been destroyed or reset.

The details

The constructor signature looks like this:

template<typename Other,typename Target>
shared_ptr(shared_ptr<Other> const& other,Target* p);

As ever, if you're constructing a shared_ptr<T> then the pointer p must be convertible to a T*, but there's no restriction on the type of Other at all. The newly constructed object shares ownership with other, so other.use_count() is increased by 1, and the value returned by get() on the new object is static_cast<T*>(p).

There's a slight nuance here: if other is an empty shared_ptr, such as a default-constructed shared_ptr, then the new shared_ptr is also empty, and has a use_count() of 0, but it has a non-NULL value if p was not NULL.

int i;
shared_ptr<int> sp(shared_ptr<X>(),&i);
assert(sp.use_count()==0);
assert(sp.get()==&i);

Whether this odd effect has any use is open to debate.

Final Thoughts

This little-known constructor is potentially very useful for passing around shared_ptr objects that reference parts of a non-trivial data structure and keep the whole data structure alive. Not everyone will need it, but for those that do it will avoid a lot of headaches.

Posted by Anthony Williams
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7 Comments

This is great! I'm designing an arena based allocator and needed something like this exactly; now I've got a neat way to create a shared pointer that increments the arena itself rather than having to use emplace_shared all over the place.

by Jason Lokerson at 04:08:10 on Saturday, 25 July 2015

I've known about this, but for some reason not understood its usefulness. Thanks for a good explanation.

by at 08:44:22 on Saturday, 25 July 2015

Le hacker news army reporting in!

by Hacker at 21:15:50 on Saturday, 25 July 2015

Le hacker news army reporting in!

by Hacker at 21:16:00 on Saturday, 25 July 2015

JUST U S T

by brendan at 15:27:08 on Sunday, 26 July 2015

An example of how std::shared_ptr aliasing constructor can be used instead of std::enable_shared_from_this: http://stackoverflow.com/a/27925567 (second piece of code in the answer).

I've found this technique quite useful for my code.

by tadam at 08:57:07 on Tuesday, 28 July 2015

Thanks Anthony! I didn't know about this - but it's very useful. It's been one of the only remaining reasons I've been keeping my own shared pointer implementation around in our code-base. I can possibly go completely std on our pointers now :-)

by Phil Nash at 09:28:58 on Tuesday, 28 July 2015

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