CODEWITHSUNDEEP
c++c++14unique-ptr
rtischer8277
rtischer8277

Reputation: 536

Why does make_unique of a struct fail when containing an std::promise as member?

I wrote the struct MyParam so that it could instantiate using any number of parameters, which in this case are an int and a bool. For encapsulation reasons, I would like for MyParams to contain its own promise so it can report when it is done doing something. But when I add the statement to the struct, it fails. As a global, though, it works fine. Here is the code:

#include <tuple>
#include <memory>
#include <future>

//std::promise< bool > donePromise; // OK: if at global level, then make_unique error goes away

template< typename... ArgsT >
struct MyParams
{
  MyParams( ArgsT... args ) : rvalRefs { std::forward_as_tuple( args... ) } {}

  std::tuple< ArgsT... > rvalRefs;
  std::promise< bool > donePromise; // causes make_unique to fail when donePromise is a member of MyParams
};

int main()
{
  int num { 33 };
  bool tf { false };
  MyParams< int, bool > myParams( num, tf ); // OK
  auto myParamsUniquePtr = std::make_unique< MyParams< int, bool > >( myParams );

  std::future< bool > doneFuture = myParams.donePromise.get_future(); // OK: when donePromise is a global

  return 0;
}

error C2280: 'MyParams<int,bool>::MyParams(const MyParams<int,bool> &)': attempting to reference a deleted function

What am I missing with regards to the promise statement as a member?

Upvotes: 0

Views: 687

Answers (1)

Praetorian
Praetorian

Reputation: 109159

std::promise is not copy-constructible. 

std::make_unique< MyParams< int, bool > >( myParams )

Above, make_unique is attempting to copy construct a MyParams< int, bool > which is ill-formed due to the presence of the promise data member. You can get the code to compile if you move construct instead.

std::make_unique< MyParams< int, bool > >( std::move(myParams) )

Upvotes: 2

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