[Boost-bugs] [Boost C++ Libraries] #12874: future<> extension constructor must be under BOOST_THREAD_PROVIDES_FUTURE_UNWRAP

Subject: [Boost-bugs] [Boost C++ Libraries] #12874: future<> extension constructor must be under BOOST_THREAD_PROVIDES_FUTURE_UNWRAP
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2017-02-28 00:00:54

#12874: future<> extension constructor must be under
BOOST_THREAD_PROVIDES_FUTURE_UNWRAP
------------------------------+----------------------
 Reporter:  Arthur O'Dwyer    |      Owner:  anthonyw
     Type:  Patches           |     Status:  new
Milestone:  To Be Determined  |  Component:  thread
  Version:  Boost 1.63.0      |   Severity:  Problem
 Keywords:                    |
------------------------------+----------------------
 In Boost 1.63.0's "boost/thread/future.hpp", we see this code:

 {{{
     inline explicit
 BOOST_THREAD_FUTURE(BOOST_THREAD_RV_REF(BOOST_THREAD_FUTURE<BOOST_THREAD_FUTURE<R>
 >) other); // EXTENSION
 }}}

 Unfortunately, the actual definition of this inline constructor is located
 under an #ifdef that may not be defined at compile time:

 {{{
 #if defined BOOST_THREAD_PROVIDES_FUTURE_UNWRAP
 // many lines snipped...
   template <typename R>
   inline
 BOOST_THREAD_FUTURE<R>::BOOST_THREAD_FUTURE(BOOST_THREAD_RV_REF(BOOST_THREAD_FUTURE<BOOST_THREAD_FUTURE<R>
 >) other)
   : base_type(other.unwrap()) {}
 // several lines snipped...
 #endif
 }}}

 The result is that this inline constructor is declared but not defined,
 which produces very confusing linker errors.

 The appropriate solution is to wrap the declaration of this constructor in

 {{{
 #if defined BOOST_THREAD_PROVIDES_FUTURE_UNWRAP
 #endif
 }}}

 No other change needs to be made, as far as I know.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/12874>
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