Forwarding to std::async - c++

Question

This is my (simplified) code where I'm trying to call async forwarding arguments:
template<typename Ret, typename ... Args>
class CallbackAsyncTask {
public:
CallbackAsyncTask() {
}
virtual ~CallbackAsyncTask() = default;
void execute( Args&& ... args ) {
execute(&CallbackAsyncTask<Ret, Args...>::onBackground, this, std::forward<Args>(args)...);
}
protected:
virtual Ret onBackground( Args& ... args ) = 0;
template<typename Fn, typename ... Argss>
void execute( Fn&& fn, Argss&& ... args ) noexcept(false) {
std::async(std::launch::async, std::forward<Fn>(fn), std::forward<Argss>(args)...);
}
};
class Child: public CallbackAsyncTask<int, int> {
public:
virtual int onBackground( int& i ) {
return i;
}
};
int main() {
Child c;
c.execute(15);
return 0;
}
I get this error:
../main.cpp: In instantiation of ‘void CallbackAsyncTask<Ret, Args>::execute(Fn&&, Argss&& ...) [with Fn = int (CallbackAsyncTask<int, int>::*)(int&); Argss = {CallbackAsyncTask<int, int>* const, int}; Ret = int; Args = {int}]’:
../main.cpp:27:98: required from ‘void CallbackAsyncTask<Ret, Args>::execute(Args&& ...) [with Ret = int; Args = {int}]’
../main.cpp:46:17: required from here
../main.cpp:33:90: error: no matching function for call to ‘async(std::launch, int (CallbackAsyncTask<int, int>::*)(int&), CallbackAsyncTask<int, int>* const, int)’
std::async(std::launch::async, std::forward<Fn>(fn), std::forward<Argss>(args)...);
^
../main.cpp:33:90: note: candidates are:
In file included from ../main.cpp:13:0:
/usr/include/c++/4.8.2/future:1532:5: note: template<class _Fn, class ... _Args> std::future<typename std::result_of<_Functor(_ArgTypes ...)>::type> std::async(std::launch, _Fn&&, _Args&& ...)
async(launch __policy, _Fn&& __fn, _Args&&... __args)
^
/usr/include/c++/4.8.2/future:1532:5: note: template argument deduction/substitution failed:
/usr/include/c++/4.8.2/future: In substitution of ‘template<class _Fn, class ... _Args> std::future<typename std::result_of<_Functor(_ArgTypes ...)>::type> std::async(std::launch, _Fn&&, _Args&& ...) [with _Fn = int (CallbackAsyncTask<int, int>::*)(int&); _Args = {CallbackAsyncTask<int, int>* const, int}]’:
../main.cpp:33:90: required from ‘void CallbackAsyncTask<Ret, Args>::execute(Fn&&, Argss&& ...) [with Fn = int (CallbackAsyncTask<int, int>::*)(int&); Argss = {CallbackAsyncTask<int, int>* const, int}; Ret = int; Args = {int}]’
../main.cpp:27:98: required from ‘void CallbackAsyncTask<Ret, Args>::execute(Args&& ...) [with Ret = int; Args = {int}]’
../main.cpp:46:17: required from here
/usr/include/c++/4.8.2/future:1532:5: error: no type named ‘type’ in ‘class std::result_of<int (CallbackAsyncTask<int, int>::*(CallbackAsyncTask<int, int>*, int))(int&)>’
../main.cpp: In instantiation of ‘void CallbackAsyncTask<Ret, Args>::execute(Fn&&, Argss&& ...) [with Fn = int (CallbackAsyncTask<int, int>::*)(int&); Argss = {CallbackAsyncTask<int, int>* const, int}; Ret = int; Args = {int}]’:
../main.cpp:27:98: required from ‘void CallbackAsyncTask<Ret, Args>::execute(Args&& ...) [with Ret = int; Args = {int}]’
../main.cpp:46:17: required from here
/usr/include/c++/4.8.2/future:1552:5: note: template<class _Fn, class ... _Args> std::future<typename std::result_of<_Functor(_ArgTypes ...)>::type> std::async(_Fn&&, _Args&& ...)
async(_Fn&& __fn, _Args&&... __args)
^
/usr/include/c++/4.8.2/future:1552:5: note: template argument deduction/substitution failed:
/usr/include/c++/4.8.2/future: In substitution of ‘template<class _Fn, class ... _Args> std::future<typename std::result_of<_Functor(_ArgTypes ...)>::type> std::async(_Fn&&, _Args&& ...) [with _Fn = std::launch; _Args = {int (CallbackAsyncTask<int, int>::*)(int&), CallbackAsyncTask<int, int>* const, int}]’:
../main.cpp:33:90: required from ‘void CallbackAsyncTask<Ret, Args>::execute(Fn&&, Argss&& ...) [with Fn = int (CallbackAsyncTask<int, int>::*)(int&); Argss = {CallbackAsyncTask<int, int>* const, int}; Ret = int; Args = {int}]’
../main.cpp:27:98: required from ‘void CallbackAsyncTask<Ret, Args>::execute(Args&& ...) [with Ret = int; Args = {int}]’
../main.cpp:46:17: required from here
/usr/include/c++/4.8.2/future:1552:5: error: no type named ‘type’ in ‘class std::result_of<std::launch(int (CallbackAsyncTask<int, int>::*)(int&), CallbackAsyncTask<int, int>*, int)>’
What I am missing? Compiler Gcc 4.8.3

Answer 1

The root cause is these two lines
virtual Ret onBackground( Args& ... args ) = 0;
virtual int onBackground( int& i )
When launching the task, the arguments are converted either directly or as if by the std::thread constructor. That one does decay_copy on the arguments when invoking the callable, so a non-const lvalue reference cannot bind to the argument.
Change the way onBackground accepts arguments. For instance, a const lvalue refernce works. See live.