post@… — Sat, 14 May 2016 08:36:15 GMT

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:

optional/optional_fwd.hpp includes config/suffix.hpp
config/suffix.hpp uses an include guard to prevent multiple inclusions of itself
config/suffix.hpp doesn't define int128_type and uint128_type because BOOST_HAS_INT128 is not defined
another boost header includes config.hpp
config.hpp includes config/compiler/gcc.hpp
config/compiler/gcc.hpp defines BOOST_HAS_INT128
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):

optional/optional_fwd.hpp includes config/suffix.hpp
config/suffix.hpp first sets BOOST_LIKELY to a no-op default
another boost header includes config.hpp
config.hpp includes config/compiler/gcc.hpp
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)

post@… — Sat, 14 May 2016 08:42:03 GMT

I posted a pull request with the (trivial) fix for this problem on Github: https://github.com/boostorg/optional/pull/19

Wed, 10 Aug 2016 19:55:56 GMT

I think this is related to ticket:12142.

akrzemi1 — Mon, 19 Sep 2016 07:42:23 GMT

A related ticket #12142 will be fixed in 1.62.0 release. Hopefully this one will be fixed as well.

status, milestone changed; resolution set

akrzemi1 — Mon, 19 Sep 2016 07:43:32 GMT

status new → closed
resolution → fixed
milestone To Be Determined → Boost 1.62.0

Boost C++ Libraries: Ticket #12179: Including boost/optional/optional_fwd.hpp without first including boost/config.hpp doesn't compile.

status, milestone changed; resolution set