# On lifetime extension and decltype(auto)

“Lifetime extension” is a weird little quirk of C++98 that is still alive and kicking in C++17. The basic idea is that you can write C++ code like this:

int main() {
const int& three = 1 + 2;
return three;
}


and it will just magically work. Sure, it looks like you’re taking a reference to a temporary, there; but actually behind the scenes the compiler will extend the lifetime of that temporary to match the scope of the reference variable to which it’s bound (namely, three).

This magic happens only when the compiler can see that you’re dealing with a temporary, a prvalue expression, of type exactly T. The magic will not happen if you are dealing with an expression of type T&&, or const T&, or anything else that might conceivably be an xvalue. Thus:

int foo(const int& x) { return x; }
int a        = foo(42);  // OK
const int& b = foo(42);  // OK

const int& bar(const int& y) { return y; }
int c        = bar(42);  // OK
const int& d = bar(42);  // ERROR!


Because of the ERROR! on this final line, I’ve always considered lifetime extension to be a pretty dangerous and arcane feature of C++, basically a bug in C++98 that now we can’t ever get rid of for historical reasons — and if you find yourself using it, you are probably doing something wrong.

Similarly, I’ve always felt that the decltype(auto) return type is basically dangerous and you should avoid using it in “normal” code. (Maybe it has a use in some arcane metaprogramming. I don’t know.)

However, the other day, while working on my Hanabi bots, I did come across a thought-provoking situation.

class Server {
public:
}
};

class GameView {
const Server *server_;
public:
}
};

void do_stuff(GameView view) {
// ...
}


Now let’s suppose that we replace this slow and heap-happy std::vector<Card> with something small and fast and trivially copyable, like my bot’s CardCounts type.

class Server {
public:
}
};


We don’t need to change any of the rest of the code. But then, why are we passing around references to trivially copyable (and small) types? Let’s just do value semantics right!

class Server {
public:
}
};


…And now we have a dangling-reference bug! GameView::discards() returns a const reference to the result of server_->discards(), which is a temporary, which means that that reference is dangling as soon as the function returns. This is horrible enough that both Clang and GCC will warn us about it.

We can adjust GameView::discards() to return by-value… but then it’ll do something inefficient (make a copy) if we ever change Server::discards() back to returning by const reference.

So this might be the fabled use-case for decltype(auto):

class GameView {
const Server *server_;
public:
}
};


This returns by-value (and does copy elision) if server_->discards() returns by-value; and it returns by-const-reference if server_->discards() returns by-const-reference. Nifty!

And the kicker: Notice that when view.discards() returns by-value, our client code in do_stuff magically continues to work — because of lifetime extension!

void do_stuff(GameView view) {
// ...
}


However, it’s true that we could replace const auto& with decltype(auto) here as well, and then we wouldn’t be relying on lifetime extension anymore.

int a = 42;
const int& foo() { return a; }

int main() {
decltype(auto) b = foo();
assert( &a == &b );
}


Does this mean you should use decltype(auto) in your own code? Probably not.

Does this mean you should love lifetime extension? Probably not.

But it made me think that maybe there’s an argument to be made in favor of both of them, sometimes. They’re no longer 100% terrible in my eyes. Just, like, 99% terrible.

Posted 2018-04-05