In praise of make_unique

Lately I’ve been working on a codebase that does a lot of this:

class Widget {
using ptr = std::shared_ptr<Widget>;
// ...
};

Widget::ptr p(new Widget(stuff));
// ...
p.reset(new Widget(otherstuff));
// ...
p.reset();


This style has advantages and disadvantages — but mostly, disadvantages. The (lone?) advantage is that there is only a single mention of std:: anywhere in this code, which means that it’s very easy to swap out std::shared_ptr for boost::shared_ptr or any other smart pointer. (Actually, in my codebase, the most likely candidate would be our hand-written smart pointer that does intrusive ref-counting and uses RCU for reclamation.)

The other day, someone on Slack mentioned a disadvantage of this style: they’d been using this style and accidentally wrote the last line above as

p.release();  // oops, meant "p.reset"


This leaks the shared_ptr instead of freeing it. And compilers don’t diagnose this mistake (because p.release(); is arguably a reasonable thing to write, if you precede it with q.take_ownership_of(p.get())).

I claim that the problem of “misspelling the name of an obscure mutator method” goes away entirely if you make a habit of not using obscure mutator methods. So, I refactor the Widget snippet this way:

class Widget {
// ...
};

auto p = std::make_shared<Widget>(stuff);
// ...
p = std::make_shared<Widget>(otherstuff);
// ...
p = nullptr;


This style loses the ability to switch to boost::shared_ptr with a one-line patch, but it gains several other advantages:

• No raw new and delete. I try to make it so that git grep -l 'new ' returns no results, or at least returns a small O(1) number of results (for example, a single use of placement new syntax deep in the guts of some utility type). If your code is littered with .reset(new ...), then you’ll never be able to find that one memory leak caused by an unmanaged new.

• Fewer allocations, because of how make_shared works.

• More readable by novice programmers; you might have to look up what reset means, but operator= is immediately intuitive (to curly-brace programmers, anyway).

• No chance of typo’ing reset as release.

And of course the same thing goes for std::unique_ptr and std::make_unique, which is why it’s so nice that we finally got std::make_unique in C++14.

The versatile unique_ptr

One little-known but very useful fact about std::unique_ptr and std::shared_ptr is that the former implicitly converts to the latter!

template<class Y, class Deleter>
shared_ptr(std::unique_ptr<Y, Deleter>&& r);


So if we have a codebase that mixes shared_ptr and unique_ptr (perhaps we’re in the process of converting the former to the latter wherever possible), it is convenient that

myptr_ = std::make_unique<T>();


always compiles, regardless of whether myptr_ is a std::shared_ptr<T> or a std::unique_ptr<T>.

Should make_widget return unique_ptr?

This means that if you have a factory function that returns a new object, you may be better off making it return unique_ptr than shared_ptr, even if you plan usually to use the result as a shared_ptr! Here are some common usage patterns for such a factory function:

std::shared_ptr<Widget> make_widget() {
return std::make_shared<WidgetImpl>(stuff);
}

class WidgetTestHarness {
std::shared_ptr<Widget> widget_;
WidgetTestHarness() {
widget_ = make_widget();
}
};

void view_widget(Widget& w);
void keep_widget(std::shared_ptr<Widget> sptr);

void manipulator() {
auto w = make_widget();
keep_widget(w);
view_widget(*w);
}


Notice (well, suppose) that our WidgetTestHarness never needs to copy its shared_ptr<Widget>. Really it just wants a member Widget widget_;, but it can’t do that because the only way to create a Widget object is via make_widget. (And notice why that is: make_widget() secretly creates a WidgetImpl object, not a base Widget!) So it would settle for holding a unique_ptr<Widget>, if it could.

Philosophically, it makes sense that make_widget() should return a unique_ptr. I mean, it’s creating a brand-new Widget. There’s no way that anyone else in the program could already hold a reference to this Widget. So the pointer returned from make_widget is, literally, a unique pointer. We are certainly tempted to represent that unique pointer using unique_ptr.

Let’s see if we can make that happen.

std::unique_ptr<Widget> make_widget() {
return std::make_unique<WidgetImpl>(stuff);  // Danger, Will Robinson!
}

class WidgetTestHarness {
std::unique_ptr<Widget> widget_;
WidgetTestHarness() {
widget_ = make_widget();
}
};

void view_widget(Widget& w);
void keep_widget(std::shared_ptr<Widget> sptr);

void manipulator() {
std::shared_ptr<Widget> w = make_widget();  // Slightly awkward.
keep_widget(w);
view_widget(*w);
}


This rewrite gets us the behavior we want — unique_ptr in WidgetTestHarness — with relatively little downside. I’ve thought of two downsides, annotated above.

Downside number one: When we convert unique_ptr<WidgetImpl> to unique_ptr<Widget>, we change the deleter as well, from default_delete<WidgetImpl> (which calls ~WidgetImpl) to default_delete<Widget> (which calls ~Widget). If ~Widget is not virtual, then we have introduced a bug. So we’d better make dang sure that our Widget has a virtual destructor, if we’re going to be using this pattern with make_widget.

I could swear that sometimes Clang is smart enough to diagnose the accidental call to Widget’s non-virtual destructor, but not in this case. (GCC also does not diagnose it, but I wouldn’t expect them to.)

Downside number two: If we keep the auto w in manipulator(), then the call to keep_widget(w) won’t compile. You can implicitly convert an rvalue unique_ptr to shared_ptr (the shared_ptr just steals ownership from the unique_ptr), but you can’t convert an lvalue unique_ptr to shared_ptr (because if we stole the ownership from w, then the following line’s view_widget(*w) would blow up with a null pointer dereference).

Therefore, we had to replace auto w with std::shared_ptr<Widget> w, which might be too much typing for some people (and makes the patch even bigger if we ever want to switch to boost::shared_ptr).

Notice that in C++17 we can write simply

std::shared_ptr w = make_widget();


which is still a lot of typing, and incidentally I’m not a fan of gratuitous CTAD, and incidentally this line won’t work on libc++ yet because they have not implemented deduction guides for shared_ptr yet. But libstdc++ has implemented them; and presumably libc++ will follow suit relatively soon. (C++17 deduction guides for the STL containers are landing in libc++ as we speak!)

Posted 2018-05-26