Re: [Boost-bugs] [Boost C++ Libraries] #12179: Including boost/optional/optional_fwd.hpp without first including boost/config.hpp doesn't compile.

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #12179: Including boost/optional/optional_fwd.hpp without first including boost/config.hpp doesn't compile.
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2016-05-14 08:36:15

#12179: Including boost/optional/optional_fwd.hpp without first including
boost/config.hpp doesn't compile.
-------------------------------+-----------------------
  Reporter:  c.d.glover@…      |      Owner:  fcacciola
      Type:  Bugs              |     Status:  new
 Milestone:  To Be Determined  |  Component:  optional
   Version:  Boost 1.61.0      |   Severity:  Problem
Resolution:                    |   Keywords:
-------------------------------+-----------------------

Comment (by post@…):

 Here's a slightly different problem, but with the same solution:

 If optional/optional_fwd.hpp is included before any boost header that
 includes config.hpp (for example optional.hpp), there are multiple
 problems. Example:

 {{{
 #include "boost/optional/optional_fwd.hpp"
 #include "boost/optional.hpp"

 int main() {
     return 0;
 }
 }}}

 The first problem is a build failure because of inconsistent defines:

 1. optional/optional_fwd.hpp includes config/suffix.hpp
 2. config/suffix.hpp uses an include guard to prevent multiple inclusions
 of itself
 3. config/suffix.hpp doesn't define int128_type and uint128_type because
 BOOST_HAS_INT128 is not defined
 4. another boost header includes config.hpp
 5. config.hpp includes config/compiler/gcc.hpp
 6. config/compiler/gcc.hpp defines BOOST_HAS_INT128
 7. config.hpp includes config/suffix.hpp, but the include guard prevents
 reevaluation of the file

 Now, we have BOOST_HAS_INT128 defined, but boost::int128_type and
 boost::uint128_type are not. If we now try to compile something that
 includes type_traits/is_integral.hpp (which optional/optional.hpp
 indirectly does), we get to this code, which doesn't compile due to
 unknown types:

 {{{
 #ifdef BOOST_HAS_INT128
 template<> struct is_integral<boost::int128_type> : public true_type{};
 template<> struct is_integral<boost::uint128_type> : public true_type{};
 #endif
 }}}

 The second class of problems are redefinition warnings. Here's an example
 (there are multiple defines that get redefined):

 1. optional/optional_fwd.hpp includes config/suffix.hpp
 2. config/suffix.hpp first sets BOOST_LIKELY to a no-op default
 3. another boost header includes config.hpp
 4. config.hpp includes config/compiler/gcc.hpp
 5. config/compiler/gcc.hpp redefines BOOST_LIKELY to work as intended ->
 redefinition warning

 As a conclusion, optional/optional_fwd.hpp should include config.hpp
 instead of config/suffix.hpp Currently, optional/optional_fwd.hpp is the
 only file in boost that includes config/suffix.hpp directly (not through
 config.hpp), so it's probably not meant to be used this way.

 (context: I'm using gcc 5.3.1 on Fedora 23)

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/12179#comment:1>
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