Re: [Boost-users] [Boost.Thread] Compile error: boost::promise<T>::set_value(const T&) doesn't exist

Subject: Re: [Boost-users] [Boost.Thread] Compile error: boost::promise<T>::set_value(const T&) doesn't exist
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2014-08-10 18:57:45

On 7 Aug 2014 at 11:28, Rob Desbois wrote:

> I'm having problems compiling with Boost.Thread's futures I can't seem
> to pass anything but a primitive type into make_ready_future. I've had
> problems with other parts of Thread (packaged_task, promise,
> unique/shared_future) that have produced horrendously unhelpful
> compile errors.
> 
> Here is a minimal testcase:
>     #include <boost/thread/future.hpp>
> 
>     struct foo
>     {
>         foo(int i_): i(i_) {}
>         int i;
>     };
> 
>     int main()
>     {
>         // A const future ref isn't much use, but I needed to prove
>         // the problem wasn't me trying to copy a unique_future
>         const boost::unique_future<foo>& fut =
> boost::make_ready_future( foo(42) );
>     }
> 
> 
> With `BOOST_THREAD_USES_MOVE` defined before including the
> Boost.Future header I get the following error with gcc 4.8.2 and Boost
> 1.55 (full output at http://pastebin.com/0sePWBWr):
> 
>     ../../deps/boost/include/boost/thread/future.hpp:3634:5: error: no
> matching function for call to `boost::promise<foo>::set_value(const
> foo&)´
>          p.set_value(boost::forward<future_value_type>(value));
>          ^
> 
> There seems to be no overload of promise::set_value() that takes a
> const lvalue reference. Looking at `promise` and `future_traits` in
> `future.hpp` it seems that the const lvalue ref overload will only
> exist when `BOOST_NO_CXX11_RVALUE_REFERENCES` is undefined. That makes
> no sense to me however...surely the const lvalue ref overload is
> needed precisely when there are no rvalue references? (Note this
> happens even if I pass a mutable lvalue ref to `make_ready_future()`).
> 
> 
> If I don't define `BOOST_THREAD_USES_MOVE` it fails compilation with
> the following error (full output at http://pastebin.com/jBwiJLeC):
> 
>     ../../deps/boost/include/boost/thread/detail/move.hpp:183:54:
> error: no matching function for call to
> boost::unique_future<foo>::unique_future(boost::unique_future<foo>)´
>     #define BOOST_THREAD_MAKE_RV_REF(RVALUE) RVALUE.move()
>                                                          ^
> 
> Have I missed something? I'm unable to switch to C++11 mode
> unfortunately, but this seems such a big problem I can only assume
> I've done something wrong.
> I still get similar compile errors trying with Clang 3.4, and also
> with Boost 1.54 (g++ or Clang).

I confirm that futures have become broken under C++03 in 1.56. Please 
file a bug report.

Niall

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