# A trivially copyable ticket for a unique_ptr

In Chandler Carruth’s CppCon 2019 talk “There Are No Zero-Cost Abstractions,” he talked a lot (not exclusively, but a lot) about the hidden performance cost of std::unique_ptr. See, unique_ptr may be the same size as a native pointer, but because it has a user-defined destructor, it is “non-trivial for purposes of ABI,” and that means it has to get passed on the stack.

For more on “trivial for purposes of ABI,” see my previous blog post [[trivial_abi]] 101” (May 2018).

It occurred to me afterward that even if you didn’t want to write your own custom [[trivial_abi]]-enabled MyUniquePtr (maybe because you valued GCC compatibility — I think GCC still hasn’t implemented that attribute), you might be able to hack around it by simply creating a trivially copyable “ticket for a unique_ptr,” in the same sense that I describe weak_ptr as “a ticket for a shared_ptr.” The important thing about a ticket is that you can’t use it directly; it doesn’t afford the user any operation except “redeem this ticket for a unique_ptr.”

It would look something like this (Godbolt):

template<class T>
struct ticket {
ticket() = default;
explicit ticket(T *p) : p_(p) {}
explicit ticket(std::unique_ptr<int>&& p) : p_(p.release()) {}
std::unique_ptr<T> redeem() && { return std::unique_ptr<T>(std::exchange(p_, nullptr)); }
private:
T *p_ = nullptr;
};


And then Chandler’s test harness, which originally looked something like this and produced 27 lines of assembly —

void bar(int*);
void baz(std::unique_ptr<int>);

void foo(std::unique_ptr<int> p) {
bar(p.get());
baz(std::move(p));
}


— would instead look something like this and produce 19 lines of assembly —

void bar(int*);
void baz(ticket<int>);

void foo(ticket<int> t) {
std::unique_ptr<int> p = std::move(t).redeem();
bar(p.get());
baz(ticket<int>(std::move(p)));
}


So what’s the catch? Well, it’s a big one. The trivially copyable ticket object by definition has a trivial destructor. So it doesn’t consider itself to “own” the heap-allocated object. If an exception is thrown during the time the heap allocation is managed only by the ticket, then the allocation will be leaked!

void use(ticket<int> t, int u);
int thrower() { throw "oops"; }

void test() {
auto p = std::make_unique<int>(42);
use(
ticket<int>(std::move(p)), // lose ownership...
thrower()  // ...and leak the allocation!
);
}


However, as Chandler himself pointed out, this is only a problem if your codebase uses exceptions at all! If you don’t use exceptions, then you don’t have this issue, and maybe the idea of a “trivially copyable ticket for a unique_ptr” might be interesting to you.

What would make this pattern actually usable, I think, would be if the language had some way to say “ABI-wise, I take a parameter of type X; but the first and only thing I’m ever going to do with that parameter is to convert it to type Y.” Something like this fantasy syntax:

void baz(ticket_view<int>);

void foo(ticket_view<int> -> std::unique_ptr<int> p) {
bar(p.get());
baz(std::move(p));
}


(Here I’ve renamed ticket to ticket_view, and given it an implicit constructor from unique_ptr, and given it an explicit conversion to unique_ptr instead of a named method redeem(). This emphasizes its similarity to string_view as a parameter-only type.)

Posted 2019-09-21