ARTICLE AD BOX
On cppreference (which I understand is not the c++ standard itself) I have read that the coroutine state contains "local variables and temporaries whose lifetime spans the current suspension point.", (emphasis mine) but I don't think I understand how this works.
Suppose I have a coroutine that looks something like this:
/*...*/ SomeCoroutine() { const auto& ref = std::string("abc"); co_yield ref.size(); // assume the coroutine really does suspend co_return ref.size(); }Before the coroutine is suspended, is ref a local variable on the stack, or is it really a member of the coroutine state (in which case I doubt it can actually be a reference)?
After the coroutine is resumed, is ref a local variable on the stack, or is it really a member of the coroutine state? If it is still a reference, is it dangling now, or did the string get copied somehow?
I think my mistake is presuming that ref is an object. ref is an alias for the materialized temporary, and its the temporary that is an object. It would make sense to me if the temporary itself is in the coroutine state all along.
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From the page you quoted on cppreference.com:
Each coroutine is associated with
...
the coroutine state, which is internal, dynamically-allocated storage (unless the allocation is optimized out), object that contains...
local variables and temporaries whose lifetime spans the current suspension point.(emphasis is mine)
In the "normal" (non-coroutine) case the following code:
const auto& ref = std::string("abc");Will extend the life of the temporary because of the bounding to the const-ref. This seems like exactly the case mentioned above (with my emphasis in bold).
Therefore there is no dangling reference in your code:
The std::string temporary object will be kept as a part of the coroutine state and the reference will refer to it.
Just to make it clear: as you suggested, the reference itself is not an object, but an alias to the temporary object.
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My faulty line of reasoning was something like "ref itself is an object" (incorrect) -> "ref itself is saved to / part of the coroutine state" -> "the coroutine state has the moral equivalent of a const std::string& ref member that the temporary binds to". The alternative of "the coroutine state has the moral equivalent of a std::string __tmp1 member instead and ref never occupies memory anywhere" makes a lot more sense.
2026-01-01T12:40:25.137Z+00:00
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