Storing std::promise objects in a std::pair - c++11

Question

I'm working on a couple of C++11 work queue classes. The first class, command_queue is a multi producer single consumer work queue. Multiple threads can post commands, and a single thread calls "wait()" and "pop_back()" in a loop to process those commands.
The second class, Actor uses command_queue and actually provides a consumer thread... besides that, the idea is that post() will return a future so that clients can either block until the command is processed, or continue running (actor also adds the idea of a result type). To implement this, I'm attempting to store std::promise's in a std::pair in the work queue. I believe I am fairly close, but i'm having a problem in the _entry_point function below... specifically, when I'm trying to get the std::pair out of the command queue I'm getting a "use of deleted function" compiler error... I'll put the actual error i'm getting from the compiler below the code (you should be able to save this to a text file and compile it yourself, it's stand alone c++11 code).
#include <mutex>
#include <condition_variable>
#include <future>
#include <list>
#include <stdio.h>
template<class T>
class command_queue
{
public:
command_queue() = default;
command_queue( const command_queue& ) = delete;
virtual ~command_queue() noexcept = default;
command_queue& operator = ( const command_queue& ) = delete;
void start()
{
std::unique_lock<std::recursive_mutex> g( _queueLock );
_started = true;
}
bool started()
{
return _started;
}
void stop()
{
std::unique_lock<std::recursive_mutex> g( _queueLock );
_started = false;
_queueCond.notify_one();
}
void post_front( const T& cmd )
{
std::unique_lock<std::recursive_mutex> g( _queueLock );
_queue.push_front( cmd );
_queueCond.notify_one();
}
void post_front( T&& cmd )
{
std::unique_lock<std::recursive_mutex> g( _queueLock );
_queue.push_front( cmd );
_queueCond.notify_one();
}
void wait()
{
std::unique_lock<std::recursive_mutex> g( _queueLock );
_queueCond.wait( g, [this](){return !this->_queue.empty() ? true : !this->_started;});
}
T pop_back()
{
std::unique_lock<std::recursive_mutex> g( _queueLock );
auto val = _queue.back();
_queue.pop_back();
return val;
}
private:
std::recursive_mutex _queueLock;
std::condition_variable_any _queueCond;
std::list<T> _queue;
bool _started = false;
};
template<class T, class U>
class actor
{
public:
actor() :
_started( false ),
_thread(),
_queue()
{
}
actor( const actor& ) = delete;
virtual ~actor() noexcept
{
if( _started )
stop();
}
actor& operator = ( const actor& ) = delete;
void start()
{
_started = true;
_queue.start();
_thread = std::thread( &actor<T,U>::_entry_point, this );
}
void stop()
{
_started = false;
_queue.stop();
_thread.join();
}
std::future<U> post( const T& cmd )
{
std::promise<U> p;
std::future<U> waiter = p.get_future();
_queue.post_front( std::pair<T,std::promise<U>>(cmd, std::move(p)) );
return waiter;
}
virtual U process( const T& cmd ) = 0;
protected:
void _entry_point()
{
while( _started )
{
_queue.wait();
if( !_started )
continue;
std::pair<T,std::promise<U>> item = _queue.pop_back();
item.second.set_value( process( item.first ) );
}
}
bool _started;
std::thread _thread;
command_queue<std::pair<T,std::promise<U>>> _queue;
};
class int_printer : public actor<int,bool>
{
public:
virtual bool process( const int& cmd ) override
{
printf("%d",cmd);
return true;
}
};
using namespace std;
int main( int argc, char* argv[] )
{
// std::promise<bool> p;
// list<std::pair<int,std::promise<bool>>> promises;
// promises.push_back( make_pair<int,std::promise<bool>>(10,std::move(p)) );
int_printer a;
a.start();
future<bool> result = a.post( 10 );
a.stop();
}
[developer#0800275b874e projects]$ g++ -std=c++11 pf.cpp -opf -lpthread
pf.cpp: In instantiation of ‘T command_queue<T>::pop_back() [with T = std::pair<int, std::promise<bool> >]’:
pf.cpp:133:65: required from ‘void actor<T, U>::_entry_point() [with T = int; U = bool]’
pf.cpp:99:9: required from ‘void actor<T, U>::start() [with T = int; U = bool]’
pf.cpp:163:13: required from here
pf.cpp:60:32: error: use of deleted function ‘constexpr std::pair<_T1, _T2>::pair(const std::pair<_T1, _T2>&) [with _T1 = int; _T2 = std::promise<bool>]’
In file included from /usr/lib/gcc/x86_64-redhat-linux/4.7.0/../../../../include/c++/4.7.0/utility:72:0,
from /usr/lib/gcc/x86_64-redhat-linux/4.7.0/../../../../include/c++/4.7.0/tuple:38,
from /usr/lib/gcc/x86_64-redhat-linux/4.7.0/../../../../include/c++/4.7.0/mutex:39,
from pf.cpp:2:
/usr/lib/gcc/x86_64-redhat-linux/4.7.0/../../../../include/c++/4.7.0/bits/stl_pair.h:119:17: note: ‘constexpr std::pair<_T1, _T2>::pair(const std::pair<_T1, _T2>&) [with _T1 = int; _T2 = std::promise<bool>]’ is implicitly deleted because the default definition would be ill-formed:
/usr/lib/gcc/x86_64-redhat-linux/4.7.0/../../../../include/c++/4.7.0/bits/stl_pair.h:119:17: error: use of deleted function ‘std::promise<_Res>::promise(const std::promise<_Res>&) [with _Res = bool]’
In file included from pf.cpp:4:0:
/usr/lib/gcc/x86_64-redhat-linux/4.7.0/../../../../include/c++/4.7.0/future:963:7: error: declared here
In file included from /usr/lib/gcc/x86_64-redhat-linux/4.7.0/../../../../include/c++/4.7.0/list:64:0,
from pf.cpp:5:

Answer 1

Promises aren't copyable (which makes sense - they represent a unique state). You need to use std::move in several places to transfer the unique ownership of the promise along.
Specifically, your home-brew queue class needs to permit moving, e.g.
auto val = std::move(_queue.back());
_queue.pop_back();
return val;

Answer 2

You protect writes to command_queue::_started with _queueLock, but not the read in command_queue::started(); this is a data race if some thread can call started while another thread is performing a modification (e.g., stop()).
Several small observations:
It doesn't make your program incorrect, but it's better to notify a condition variable outside the mutex. If you notify with the mutex held, another core may waste a microsecond or two scheduling a waiting thread to run only to immediately block on the mutex.
Your post_front(T&&) is copying the passed item into the queue due to missing a std::move:
_queue.push_front( cmd );
must be
_queue.push_front( std::move( cmd ) );
if you want it to actually be moved into the queue.
The predicate for the condition variable wait could be simplified from
[this](){return !this->_queue.empty() ? true : !this->_started;}
to
[this]{return !_queue.empty() || !_started;}
None of the command_queue member functions call other command_queue functions, so you could use a plain std::mutex instead of std::recursive_mutex and std::condition_variable instead of std::condition_variable_any.
You could use std::lock_guard<std::mutex> instead of std::unique_lock<std::mutex> to lock the mutex in every member function except wait. It's ever-so-slightly lighter weight.
You have the traditional pop exception-safety issue: If the selected move/copy constructor for T fails with an exception when returning from pop_back after modifying the queue, that element is lost. The way you've written the function makes this occurrence extremely unlikely, since
auto val = _queue.back();
_queue.pop_back();
return val;
(or after Kerrek's fix)
auto val = std::move(_queue.back());
_queue.pop_back();
return val;
should qualify for copy elision with a decent compiler, constructing the returned object in-place before the pop_back happens. Just be aware that if future changes impede copy elision you'll introduce the exception safety problem. You can avoid the issue altogether by passing a T& or optional<T>& as a parameter and move assigning the result to that parameter.
actor::_started is unnecessary since it's effectively a proxy for actor::_queue::_started.