When a regular, non-void function flows off the end of its body without returning a value, the behavior is undefined. With a
coroutine, when flowing off the end of its body, return_void() is invoked on the promise for the said coroutine. If such invocation is
not possible (e.g., because the function is not defined), the behavior is undefined.
In other words, a coroutine should either:
- have all its execution paths reach a
co_return statement or throw an exception;
- or its promise type should provide
return_void().
This rule raises an issue on coroutines that do not meet the above criteria.
Noncompliant code example
struct IsPrimeTask {
struct promise_type {
// ... no return_void() definition ...
void return_value(bool answer) { /* ... */ }
};
// ...
};
IsPrimeTask isPrime(long n) {
std::optional<bool> result = co_await Oracle::IsPrime(n);
if (result.has_value()) {
co_return result.value();
}
// Noncompliant
}
struct UploadFileTask {
struct promise_type {
// No return_void() definition.
// ...
};
// ...
};
UploadFileTask upload(ServerHandle server, File file) {
co_await server.transfert(file);
// Noncompliant
}
Compliant solution
enum class Tristate { TRUE, FALSE, UNKNOWN };
Tristate toTristate(bool value);
struct IsPrimeTask {
struct promise_type {
// ...
void return_value(Tristate answer) { /* ... */ }
};
// ...
};
IsPrimeTask isPrime(long n) {
std::optional<bool> result = co_await Oracle::IsPrime(n);
if (result.has_value()) {
co_return toTristate(result.value());
}
co_return Tristate::UNKNOWN;
}
struct UploadFileTask {
struct promise_type {
void return_void() { /* ... */ }
// ...
};
// ...
};
UploadFileTask upload(ServerHandle server, File file) {
co_await server.transfert(file);
}
