Copy constructor gets called for return value of continuation of folly Future

[sanjosh]

See attached file
copy_move.txt

Any idea how I can prevent a copy of the return type which is being made when the continuation of a Future executes ? I looked at the tests in folly/futures/test but couldn't find anything relevant

this is the stack when the copy constructor is called
` where

#0 MyData::MyData (this=0x7fffffffce60, other=...) at copy_move.cpp:14

#1 0x000000000040b5c1 in <lambda(int)>::operator()(int) const (__closure=0x6030000008e0) at copy_move.cpp:32

#2 0x000000000040bba5 in folly::detail::CoreCallbackState<MyData, func()::<lambda(int)> >::invoke(<unknown type in /home/sandeep/a.out, CU 0xdf8, DIE 0x25b4e>) (this=0x6030000008e0,
args#0=<unknown type in /home/sandeep/a.out, CU 0xdf8, DIE 0x25b4e>)
at /usr/local/include/folly/futures/Future-inl.h:85

#3 0x000000000040bc8a in folly::Future::<lambda(folly::Try&&)>::<lambda()>::operator()(void) const (
__closure=0x7fffffffd000) at /usr/local/include/folly/futures/Future-inl.h:243

#4 0x000000000040cbd7 in folly::makeTryWith<folly::Future::thenImplementation(F&&, folly::detail::argResult<isTry, F, Args ...>)::<lambda(folly::Try&&)> mutable [with F = func()::<lambda(int)>; R = folly::detail::callableResult<int, func()::<lambda(int)> >; bool isTry = false; Args = {int&&}; T = int]::<lambda()> >(<unknown type in /home/sandeep/a.out, CU 0xdf8, DIE 0x2503a>) (f=<unknown type in /home/sandeep/a.out, CU 0xdf8, DIE 0x2503a>)
at /usr/local/include/folly/Try-inl.h:151

#5 0x000000000040be83 in folly::Future::<lambda(folly::Try&&)>::operator()(<unknown type in /home/sandeep/a.out, CU 0xdf8, DIE 0x21ad0>) (__closure=0x6030000008e0,
t=<unknown type in /home/sandeep/a.out, CU 0xdf8, DIE 0x21ad0>)
at /usr/local/include/folly/futures/Future-inl.h:242

#6 0x000000000040db1a in folly::detail::function::FunctionTraits<void(folly::Try&&)>::callBig<folly::Future::thenImplementation(F&&, folly::detail::argResult<isTry, F, Args ...>) [with F = func()::<lambda(int)>; R = folly::detail::callableResult<int, func()::<lambda(int)> >; bool isTry = false; Args = {int&&}; T = int]::<lambda(folly::Try&&)> >(folly::detail::function::Data &, <unknown type in /home/sandeep/a.out, CU 0xdf8, DIE 0x20912>) (
p=..., args#0=<unknown type in /home/sandeep/a.out, CU 0xdf8, DIE 0x20912>)
at /usr/local/include/folly/Function.h:305

#7 0x0000000000418033 in folly::detail::function::FunctionTraits<void (folly::Try&&)>::operator()(folly::Try&&) (this=0x610000000040, args#0=<unknown type in /home/sandeep/a.out, CU 0xdf8, DIE 0x21636>)
at /usr/local/include/folly/Function.h:314

#8 0x0000000000415f17 in folly::detail::Core::doCallback (this=0x610000000040)
at /usr/local/include/folly/futures/detail/Core.h:388

#9 0x0000000000414208 in folly::detail::Core::maybeCallback()::{lambda()#2}::operator()() const (
__closure=0x7fffffffd7c0) at /usr/local/include/folly/futures/detail/Core.h:317

#10 0x000000000041644f in folly::detail::FSMfolly::detail::State::updateState<folly::detail::Core::maybeCallback()::{lambda()#1}, folly::detail::Core::maybeCallback()::{lambda()#2}>(folly::detail::State, folly::detail::State, folly::detail::Core::maybeCallback()::{lambda()#1} const&, folly::detail::Core::maybeCallback()::{lambda()#2} const&) (this=0x6100000000a8, A=folly::detail::State::Armed, B=folly::detail::State::Done, `

[yfeldblum]

You need to use mutable.

Instead of:

MyData e;
return folly::makeFuture().then([e = std::move(e)]() { 
  return e; // this calls copy ctor
});

This:

MyData e;
return folly::makeFuture().then([e = std::move(e)]() mutable { 
  return e; // this calls copy ctor
});

[sanjosh]

@yfeldblum , mutable didn't work. Copy constructor still gets called

[yfeldblum]

[e = std::move(e)]() mutable {
  return std::move(e);
}

[sanjosh]

worked! thanks a lot, especially given its midnight in California :-)