Why Can't OperationState be Move-Only? #1588
Description
Hi everyone, I want to make a one-shot move-only sender, so I make connect() only applies to rvalue by adding && after, which is fine.
template <class Receiver>
Operation<Receiver> connect(Receiver&& receiver) && noexcept {
return Operation(std::move(receiver));
}Then, intutively, I also make OperationState move-only, because I want it to be started only once.
void start() && noexcept {
receiver.set_value("hello");
}But it won't compile, the error says it doesn't satisfies the OperationState concept.
/opt/compiler-explorer/libs/stdexec/trunk/include/stdexec/__detail/__operation_states.hpp:63:29: in requirements with '_Op& __op' [with _Op = MySender::Operation<stdexec::__sync_wait::__receiver<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >::__t>]
/opt/compiler-explorer/libs/stdexec/trunk/include/stdexec/__detail/__operation_states.hpp:63:65: note: the required expression 'stdexec::start(__op)' is invalid
63 | && requires(_Op &__op) { stdexec::start(__op); };
| ~~~~~~~~~~~~~~^~~~~~
if I remove the && after start(), it will compile.
So Why Can't OperationState be move-only? Could anyone help me? Thanks in advance!
Full code
compiler explorer: https://godbolt.org/z/n58T9Mv45
#include <string>
#include "stdexec/__detail/__execution_fwd.hpp"
#include "stdexec/__detail/__sync_wait.hpp"
struct MySender : stdexec::sender_t {
using completion_signatures = stdexec::completion_signatures<stdexec::set_value_t(std::string)>;
template <class Receiver>
struct Operation {
Receiver receiver;
void start() && noexcept { // <- Pay attention to the '&&' here
// if I remove it, it will compile
receiver.set_value("hello");
}
};
template <class Receiver>
Operation<Receiver> connect(Receiver&& receiver) && noexcept {
return Operation(std::move(receiver));
}
};
int main() { stdexec::sync_wait(MySender()); }