Coroutines: Do co_yielded string_views dangle?

Question

I want to mix up the co_yielding string literals and std::strings

Generator<std::string_view> range(int first, const int last) {
    while (first < last) {
        char ch = first++;
        co_yield " | ";
        co_yield std::string{ch, ch, ch};
    }
}

However, I'm wondering about the lifetime of the std::string?

Maybe it's safe if you know you are going to consume the string_view immediately?

for(auto sv : range(65, 91))
   std::cout << sv;

https://godbolt.org/z/d5eoP9aTE

You could make it safe like this

Generator<std::string_view> range(int first, const int last) {
    std::string result;
    while (first < last) {
        char ch = first++;
        co_yield " | ";
        result = std::string{ch, ch, ch};
        co_yield result;
    }
}

If it were stored as a local `std::string` in the coro frame, I’d say it’s fine. I’m guessing this isn’t though. But I don’t know. — Ben, Dec 02 '22 at 00:22

score 6 · Accepted Answer · answered Dec 02 '22 at 00:29

co_yield is a fancy form of co_await. Both of these are expressions. And therefore, they follow the rules of expressions. Temporaries manifested as part of the evaluation of the expression will continue to exist until the completion of the entire expression.

co_await expressions do not complete until after the coroutine resumes. This is important, as you often co_await on prvalues, so if you do something as simple as co_await some_function(), you need the return value of some_function to continue to exist, as its await_resume function needs to be able to be called.

As previously stated, co_yield is just a fancy form of co_await. So the rules still apply. So any string_view objects returned by your generator will point to valid values between calls to resume the coroutine.

*will point to valid values **between calls to resume** the coroutine* - This is extremely important. It means that if one, for example, stores the yielded string_view's into a vector, then all they will get is a "vector of UB" :D — Not a real meerkat, Dec 02 '22 at 00:34

Coroutines: Do co_yielded string_views dangle?

1 Answers1