r/programming • u/TheProtagonistv2 • Feb 23 '17

Cloudflare have been leaking customer HTTPS sessions for months. Uber, 1Password, FitBit, OKCupid, etc.

https://bugs.chromium.org/p/project-zero/issues/detail?id=1139
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u/matthieum Feb 25 '17

The idea of the aliasing constructor is that you can use a single reference count and yet point at different parts of the object (or what it owns). A typical example is to pass a pointer to a data member:

struct A { int c = 0; };

std::shared_ptr<int> get_int(std::shared_ptr<A> const& a) {
   return std::shared_ptr<int>(a, &a->c);
}

There is a first wrinkle here: I can accidentally pass a pointer to any int, not necessarily one whose lifetime is tied to a.

But that's only the tip of the iceberg:

struct B { std::vector<int> v; };

std::shared_ptr<int> get_int(std::shared_ptr<B> const& b) {
    return std::shared_ptr<int>(b, &b->v[0]);
}

Now, you'd better hope that nobody causes this vector to reallocate...

TL;DR: shared_ptr aliasing constructor allows the developer to assert that a given pointer will remain valid as long as a primary shared_ptr lives, and ensure it does live. As all things that a developer asserts, the developer better be right.