[webkit-dev] Set once pointer types and static analysis

[Ryosuke Niwa]

Hi all,

In WebKit, it’s fairly common to write a member variable as RefPtr or std::unique_ptr that later gets lazily initialized to some value but never unset or assigned of a different value after that.

e.g.

class Foo {
    Bar& bar() {
        if (!m_bar)
            m_bar = Bar::create();
        return *m_bar;
    }
    Ref<Bar> protectedBar() { return bar(); }

    RefPtr<Bar> m_bar;
}

Assuming there is no other code modifying m_bar, foo->bar()->method() is always safe to call even if method wasn’t a trivial function. Right now, static analyzer doesn’t recognize this pattern so we’d be forced to write code like this: foo->protectedBar()->method() where ensureProtectedBar is a wrapper around ensureBar which returns Ref<Bar>.

A suggestion was made that static analyzer can recognize this patten. Specifically, if we introduced a new smart pointer types that only allow setting the value once, static analyzer can allow foo->bar()->method()and avoid ref-churn in some cases:

e.g.

class Foo {
    Bar& bar() {
        if (!m_bar)
            m_bar = Bar::create();
        return *m_bar;
    }

    SetOnceRefPtr<Bar> m_bar;
}

SetOnceRefPtr::operator=(T*) release asserts that m_ptr isn’t set, and doesn’t have a move constructor, operator=(nullptr_t), leakRef(), releaseNonNull(), etc… which can override the value of m_ptr after setting the value via operator= or in constructor.

We could create various variants: SetOnceRef, SetOnceUniquePtr, SetOnceUniqueRef.

Do you think this will be useful? 

- R. Niwa

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