Can co_yield return nothing?

Question

With the new C++ coroutines, is it possible to define a coroutine class that I can use to yield from functions without returning a value? Or does yield always have to return something? I'm just interested in the yielding functionality, not a yielded value.

I want this:

Coro Fn() {
   if (...) co_yield;

    co_return;
}

Not this:

Coro Fn() {
   if (...) co_yield dummyValue;

    co_return dymmyValue;
}

Answer 1

No.

Per [expr.yield] the grammar of a yield expression (co_yield) looks like this:

yield-expression:
   co_yield assignment-expression
   co_yield braced-init-list

You can yield a braced-init-list, which, along with an empty class, could be useful:

#include <generator>

struct empty{};
std::generator<empty> foo() {
    for (int i = 0; i < 10; ++i)
        co_yield {};
}

int main()
{
    foo();
}

Demo (gcc, C++23)