[ovs-dev] [PATCH 4/5] Automatically verify that OVS header files work OK in C++ also.

Russell Bryant russell at ovn.org
Mon Jul 31 20:44:10 UTC 2017


On Mon, Jul 31, 2017 at 4:33 PM, Ben Pfaff <blp at ovn.org> wrote:
> On Mon, Jul 31, 2017 at 01:05:32PM -0700, Ben Pfaff wrote:
>> On Mon, Jul 31, 2017 at 03:50:59PM -0400, Russell Bryant wrote:
>> > On Sun, Jul 30, 2017 at 10:54 PM, Ben Pfaff <blp at ovn.org> wrote:
>> > > This should help address a recurring problem.
>> > >
>> > > Signed-off-by: Ben Pfaff <blp at ovn.org>
>> > > ---
>> > >  .travis.yml                     |  1 +
>> > >  Makefile.am                     |  1 +
>> > >  configure.ac                    |  2 ++
>> > >  include/openvswitch/automake.mk | 14 ++++++++++++++
>> > >  m4/openvswitch.m4               | 21 +++++++++++++++++++++
>> > >  5 files changed, 39 insertions(+)
>> >
>> > Why does this patch depend on libboost?  It looks like it's only used
>> > when building a test C++ program in configure.  We could build a test
>> > C++ program without the dependency, right?
>>
>> It's because of include/openvswitch/compiler.h, which has:
>>
>>     #elif defined(__cplusplus)
>>     #include <boost/static_assert.hpp>
>>     #define BUILD_ASSERT BOOST_STATIC_ASSERT
>>     #define BUILD_ASSERT_DECL BOOST_STATIC_ASSERT
>>

Ah, sorry ...

>> We could probably define our own C++-compatible static assert without
>> boost.  It looks like C++11 and later has a built-in static_assert:
>>     http://en.cppreference.com/w/cpp/language/static_assert
>>
>> Any idea whether it's reasonable to assume C++11 support these days?

I don't know, but that seems pretty reasonable to me.

I suppose if it becomes an issue, we could re-add the boost version as
an optional dependency that would get used only if C++11 wasn't
available?

Acked-by: Russell Bryant <russell at ovn.org>


>
> Here's a version of the patch that gets rid of the boost dependency.  It
> adds a C++11 dependency instead.
>
> --8<--------------------------cut here-------------------------->8--
>
> From: Ben Pfaff <blp at ovn.org>
> Date: Mon, 31 Jul 2017 13:31:43 -0700
> Subject: [PATCH] Automatically verify that OVS header files work OK in C++
>  also.
>
> This should help address a recurring problem.
>
> Signed-off-by: Ben Pfaff <blp at ovn.org>
> ---
>  Makefile.am                        |   1 +
>  configure.ac                       |   2 +
>  include/openflow/openflow-common.h |   3 +-
>  include/openvswitch/automake.mk    |  14 +
>  include/openvswitch/compiler.h     |   5 +-
>  m4/ax_cxx_compile_stdcxx.m4        | 982 +++++++++++++++++++++++++++++++++++++
>  m4/openvswitch.m4                  |  16 +
>  7 files changed, 1018 insertions(+), 5 deletions(-)
>  create mode 100644 m4/ax_cxx_compile_stdcxx.m4
>
> diff --git a/Makefile.am b/Makefile.am
> index 9679379defef..373ef6e9189f 100644
> --- a/Makefile.am
> +++ b/Makefile.am
> @@ -108,6 +108,7 @@ MAN_ROOTS =
>  noinst_DATA =
>  noinst_HEADERS =
>  lib_LTLIBRARIES =
> +noinst_LTLIBRARIES =
>  noinst_man_MANS =
>  noinst_PROGRAMS =
>  noinst_SCRIPTS =
> diff --git a/configure.ac b/configure.ac
> index 194c4b92ee34..093b6ffed50e 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -23,6 +23,7 @@ AM_INIT_AUTOMAKE([tar-pax])
>
>  AC_PROG_CC_C99
>  AM_PROG_CC_C_O
> +AC_PROG_CXX
>  AC_PROG_CPP
>  AC_PROG_MKDIR_P
>  AC_PROG_FGREP
> @@ -135,6 +136,7 @@ OVS_CHECK_POSIX_AIO
>  OVS_CHECK_PTHREAD_SET_NAME
>  OVS_CHECK_LINUX_HOST
>  OVS_LIBTOOL_VERSIONS
> +OVS_CHECK_CXX
>  AX_FUNC_POSIX_MEMALIGN
>
>  OVS_CHECK_INCLUDE_NEXT([stdio.h string.h])
> diff --git a/include/openflow/openflow-common.h b/include/openflow/openflow-common.h
> index 410f392d20cd..5f1e22589ced 100644
> --- a/include/openflow/openflow-common.h
> +++ b/include/openflow/openflow-common.h
> @@ -60,8 +60,7 @@
>          extern int (*build_assert(void))[ sizeof(struct {               \
>                      unsigned int build_assert_failed : (EXPR) ? 1 : -1; })]
>  #else /* __cplusplus */
> -#include <boost/static_assert.hpp>
> -#define OFP_ASSERT BOOST_STATIC_ASSERT
> +#define OFP_ASSERT(EXPR) static_assert(EXPR, "assertion failed")
>  #endif /* __cplusplus */
>
>  /* Version number:
> diff --git a/include/openvswitch/automake.mk b/include/openvswitch/automake.mk
> index 699d9d74ecd0..74b31acfa66b 100644
> --- a/include/openvswitch/automake.mk
> +++ b/include/openvswitch/automake.mk
> @@ -30,3 +30,17 @@ openvswitchinclude_HEADERS = \
>         include/openvswitch/version.h \
>         include/openvswitch/vconn.h \
>         include/openvswitch/vlog.h
> +
> +if HAVE_CXX
> +# OVS does not use C++ itself, but it provides public header files
> +# that a C++ compiler should accept, so when --enable-Werror is in
> +# effect and a C++ compiler is available, we build a C++ source file
> +# that #includes all the public headers, as a way to ensure that they
> +# are acceptable as C++.
> +noinst_LTLIBRARIES += include/openvswitch/libcxxtest.la
> +nodist_include_openvswitch_libcxxtest_la_SOURCES = include/openvswitch/cxxtest.cc
> +include/openvswitch/cxxtest.cc: include/openvswitch/automake.mk
> +       $(AM_V_GEN)for header in $(openvswitchinclude_HEADERS); do      \
> +         echo $$header;                                                \
> +       done | sed 's,^include/\(.*\)$$,#include <\1>,' > $@
> +endif
> diff --git a/include/openvswitch/compiler.h b/include/openvswitch/compiler.h
> index 0dc8636add33..0076ad763578 100644
> --- a/include/openvswitch/compiler.h
> +++ b/include/openvswitch/compiler.h
> @@ -244,9 +244,8 @@
>  #define BUILD_ASSERT(EXPR) ((void) 0)
>  #define BUILD_ASSERT_DECL(EXPR) extern int (*build_assert(void))[1]
>  #elif defined(__cplusplus)
> -#include <boost/static_assert.hpp>
> -#define BUILD_ASSERT BOOST_STATIC_ASSERT
> -#define BUILD_ASSERT_DECL BOOST_STATIC_ASSERT
> +#define BUILD_ASSERT(EXPR) static_assert(EXPR, "assertion failed")
> +#define BUILD_ASSERT_DECL(EXPR) static_assert(EXPR, "assertion failed")
>  #elif (__GNUC__ * 256 + __GNUC_MINOR__ >= 0x403 \
>         || __has_extension(c_static_assert))
>  #define BUILD_ASSERT_DECL(EXPR) _Static_assert(EXPR, #EXPR)
> diff --git a/m4/ax_cxx_compile_stdcxx.m4 b/m4/ax_cxx_compile_stdcxx.m4
> new file mode 100644
> index 000000000000..5032bba8091d
> --- /dev/null
> +++ b/m4/ax_cxx_compile_stdcxx.m4
> @@ -0,0 +1,982 @@
> +# ===========================================================================
> +#  https://www.gnu.org/software/autoconf-archive/ax_cxx_compile_stdcxx.html
> +# ===========================================================================
> +#
> +# SYNOPSIS
> +#
> +#   AX_CXX_COMPILE_STDCXX(VERSION, [ext|noext], [mandatory|optional])
> +#
> +# DESCRIPTION
> +#
> +#   Check for baseline language coverage in the compiler for the specified
> +#   version of the C++ standard.  If necessary, add switches to CXX and
> +#   CXXCPP to enable support.  VERSION may be '11' (for the C++11 standard)
> +#   or '14' (for the C++14 standard).
> +#
> +#   The second argument, if specified, indicates whether you insist on an
> +#   extended mode (e.g. -std=gnu++11) or a strict conformance mode (e.g.
> +#   -std=c++11).  If neither is specified, you get whatever works, with
> +#   preference for an extended mode.
> +#
> +#   The third argument, if specified 'mandatory' or if left unspecified,
> +#   indicates that baseline support for the specified C++ standard is
> +#   required and that the macro should error out if no mode with that
> +#   support is found.  If specified 'optional', then configuration proceeds
> +#   regardless, after defining HAVE_CXX${VERSION} if and only if a
> +#   supporting mode is found.
> +#
> +# LICENSE
> +#
> +#   Copyright (c) 2008 Benjamin Kosnik <bkoz at redhat.com>
> +#   Copyright (c) 2012 Zack Weinberg <zackw at panix.com>
> +#   Copyright (c) 2013 Roy Stogner <roystgnr at ices.utexas.edu>
> +#   Copyright (c) 2014, 2015 Google Inc.; contributed by Alexey Sokolov <sokolov at google.com>
> +#   Copyright (c) 2015 Paul Norman <penorman at mac.com>
> +#   Copyright (c) 2015 Moritz Klammler <moritz at klammler.eu>
> +#   Copyright (c) 2016 Krzesimir Nowak <qdlacz at gmail.com>
> +#
> +#   Copying and distribution of this file, with or without modification, are
> +#   permitted in any medium without royalty provided the copyright notice
> +#   and this notice are preserved.  This file is offered as-is, without any
> +#   warranty.
> +
> +#serial 7
> +
> +dnl  This macro is based on the code from the AX_CXX_COMPILE_STDCXX_11 macro
> +dnl  (serial version number 13).
> +
> +AX_REQUIRE_DEFINED([AC_MSG_WARN])
> +AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
> +  m4_if([$1], [11], [ax_cxx_compile_alternatives="11 0x"],
> +        [$1], [14], [ax_cxx_compile_alternatives="14 1y"],
> +        [$1], [17], [ax_cxx_compile_alternatives="17 1z"],
> +        [m4_fatal([invalid first argument `$1' to AX_CXX_COMPILE_STDCXX])])dnl
> +  m4_if([$2], [], [],
> +        [$2], [ext], [],
> +        [$2], [noext], [],
> +        [m4_fatal([invalid second argument `$2' to AX_CXX_COMPILE_STDCXX])])dnl
> +  m4_if([$3], [], [ax_cxx_compile_cxx$1_required=true],
> +        [$3], [mandatory], [ax_cxx_compile_cxx$1_required=true],
> +        [$3], [optional], [ax_cxx_compile_cxx$1_required=false],
> +        [m4_fatal([invalid third argument `$3' to AX_CXX_COMPILE_STDCXX])])
> +  AC_LANG_PUSH([C++])dnl
> +  ac_success=no
> +  AC_CACHE_CHECK(whether $CXX supports C++$1 features by default,
> +  ax_cv_cxx_compile_cxx$1,
> +  [AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_testbody_$1])],
> +    [ax_cv_cxx_compile_cxx$1=yes],
> +    [ax_cv_cxx_compile_cxx$1=no])])
> +  if test x$ax_cv_cxx_compile_cxx$1 = xyes; then
> +    ac_success=yes
> +  fi
> +
> +  m4_if([$2], [noext], [], [dnl
> +  if test x$ac_success = xno; then
> +    for alternative in ${ax_cxx_compile_alternatives}; do
> +      switch="-std=gnu++${alternative}"
> +      cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx$1_$switch])
> +      AC_CACHE_CHECK(whether $CXX supports C++$1 features with $switch,
> +                     $cachevar,
> +        [ac_save_CXX="$CXX"
> +         CXX="$CXX $switch"
> +         AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_testbody_$1])],
> +          [eval $cachevar=yes],
> +          [eval $cachevar=no])
> +         CXX="$ac_save_CXX"])
> +      if eval test x\$$cachevar = xyes; then
> +        CXX="$CXX $switch"
> +        if test -n "$CXXCPP" ; then
> +          CXXCPP="$CXXCPP $switch"
> +        fi
> +        ac_success=yes
> +        break
> +      fi
> +    done
> +  fi])
> +
> +  m4_if([$2], [ext], [], [dnl
> +  if test x$ac_success = xno; then
> +    dnl HP's aCC needs +std=c++11 according to:
> +    dnl http://h21007.www2.hp.com/portal/download/files/unprot/aCxx/PDF_Release_Notes/769149-001.pdf
> +    dnl Cray's crayCC needs "-h std=c++11"
> +    for alternative in ${ax_cxx_compile_alternatives}; do
> +      for switch in -std=c++${alternative} +std=c++${alternative} "-h std=c++${alternative}"; do
> +        cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx$1_$switch])
> +        AC_CACHE_CHECK(whether $CXX supports C++$1 features with $switch,
> +                       $cachevar,
> +          [ac_save_CXX="$CXX"
> +           CXX="$CXX $switch"
> +           AC_COMPILE_IFELSE([AC_LANG_SOURCE([_AX_CXX_COMPILE_STDCXX_testbody_$1])],
> +            [eval $cachevar=yes],
> +            [eval $cachevar=no])
> +           CXX="$ac_save_CXX"])
> +        if eval test x\$$cachevar = xyes; then
> +          CXX="$CXX $switch"
> +          if test -n "$CXXCPP" ; then
> +            CXXCPP="$CXXCPP $switch"
> +          fi
> +          ac_success=yes
> +          break
> +        fi
> +      done
> +      if test x$ac_success = xyes; then
> +        break
> +      fi
> +    done
> +  fi])
> +  AC_LANG_POP([C++])
> +  if test x$ax_cxx_compile_cxx$1_required = xtrue; then
> +    if test x$ac_success = xno; then
> +      AC_MSG_ERROR([*** A compiler with support for C++$1 language features is required.])
> +    fi
> +  fi
> +  if test x$ac_success = xno; then
> +    HAVE_CXX$1=0
> +    AC_MSG_NOTICE([No compiler with C++$1 support was found])
> +  else
> +    HAVE_CXX$1=1
> +    AC_DEFINE(HAVE_CXX$1,1,
> +              [define if the compiler supports basic C++$1 syntax])
> +  fi
> +  AC_SUBST(HAVE_CXX$1)
> +  m4_if([$1], [17], [AC_MSG_WARN([C++17 is not yet standardized, so the checks may change in incompatible ways anytime])])
> +])
> +
> +
> +dnl  Test body for checking C++11 support
> +
> +m4_define([_AX_CXX_COMPILE_STDCXX_testbody_11],
> +  _AX_CXX_COMPILE_STDCXX_testbody_new_in_11
> +)
> +
> +
> +dnl  Test body for checking C++14 support
> +
> +m4_define([_AX_CXX_COMPILE_STDCXX_testbody_14],
> +  _AX_CXX_COMPILE_STDCXX_testbody_new_in_11
> +  _AX_CXX_COMPILE_STDCXX_testbody_new_in_14
> +)
> +
> +m4_define([_AX_CXX_COMPILE_STDCXX_testbody_17],
> +  _AX_CXX_COMPILE_STDCXX_testbody_new_in_11
> +  _AX_CXX_COMPILE_STDCXX_testbody_new_in_14
> +  _AX_CXX_COMPILE_STDCXX_testbody_new_in_17
> +)
> +
> +dnl  Tests for new features in C++11
> +
> +m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_11], [[
> +
> +// If the compiler admits that it is not ready for C++11, why torture it?
> +// Hopefully, this will speed up the test.
> +
> +#ifndef __cplusplus
> +
> +#error "This is not a C++ compiler"
> +
> +#elif __cplusplus < 201103L
> +
> +#error "This is not a C++11 compiler"
> +
> +#else
> +
> +namespace cxx11
> +{
> +
> +  namespace test_static_assert
> +  {
> +
> +    template <typename T>
> +    struct check
> +    {
> +      static_assert(sizeof(int) <= sizeof(T), "not big enough");
> +    };
> +
> +  }
> +
> +  namespace test_final_override
> +  {
> +
> +    struct Base
> +    {
> +      virtual void f() {}
> +    };
> +
> +    struct Derived : public Base
> +    {
> +      virtual void f() override {}
> +    };
> +
> +  }
> +
> +  namespace test_double_right_angle_brackets
> +  {
> +
> +    template < typename T >
> +    struct check {};
> +
> +    typedef check<void> single_type;
> +    typedef check<check<void>> double_type;
> +    typedef check<check<check<void>>> triple_type;
> +    typedef check<check<check<check<void>>>> quadruple_type;
> +
> +  }
> +
> +  namespace test_decltype
> +  {
> +
> +    int
> +    f()
> +    {
> +      int a = 1;
> +      decltype(a) b = 2;
> +      return a + b;
> +    }
> +
> +  }
> +
> +  namespace test_type_deduction
> +  {
> +
> +    template < typename T1, typename T2 >
> +    struct is_same
> +    {
> +      static const bool value = false;
> +    };
> +
> +    template < typename T >
> +    struct is_same<T, T>
> +    {
> +      static const bool value = true;
> +    };
> +
> +    template < typename T1, typename T2 >
> +    auto
> +    add(T1 a1, T2 a2) -> decltype(a1 + a2)
> +    {
> +      return a1 + a2;
> +    }
> +
> +    int
> +    test(const int c, volatile int v)
> +    {
> +      static_assert(is_same<int, decltype(0)>::value == true, "");
> +      static_assert(is_same<int, decltype(c)>::value == false, "");
> +      static_assert(is_same<int, decltype(v)>::value == false, "");
> +      auto ac = c;
> +      auto av = v;
> +      auto sumi = ac + av + 'x';
> +      auto sumf = ac + av + 1.0;
> +      static_assert(is_same<int, decltype(ac)>::value == true, "");
> +      static_assert(is_same<int, decltype(av)>::value == true, "");
> +      static_assert(is_same<int, decltype(sumi)>::value == true, "");
> +      static_assert(is_same<int, decltype(sumf)>::value == false, "");
> +      static_assert(is_same<int, decltype(add(c, v))>::value == true, "");
> +      return (sumf > 0.0) ? sumi : add(c, v);
> +    }
> +
> +  }
> +
> +  namespace test_noexcept
> +  {
> +
> +    int f() { return 0; }
> +    int g() noexcept { return 0; }
> +
> +    static_assert(noexcept(f()) == false, "");
> +    static_assert(noexcept(g()) == true, "");
> +
> +  }
> +
> +  namespace test_constexpr
> +  {
> +
> +    template < typename CharT >
> +    unsigned long constexpr
> +    strlen_c_r(const CharT *const s, const unsigned long acc) noexcept
> +    {
> +      return *s ? strlen_c_r(s + 1, acc + 1) : acc;
> +    }
> +
> +    template < typename CharT >
> +    unsigned long constexpr
> +    strlen_c(const CharT *const s) noexcept
> +    {
> +      return strlen_c_r(s, 0UL);
> +    }
> +
> +    static_assert(strlen_c("") == 0UL, "");
> +    static_assert(strlen_c("1") == 1UL, "");
> +    static_assert(strlen_c("example") == 7UL, "");
> +    static_assert(strlen_c("another\0example") == 7UL, "");
> +
> +  }
> +
> +  namespace test_rvalue_references
> +  {
> +
> +    template < int N >
> +    struct answer
> +    {
> +      static constexpr int value = N;
> +    };
> +
> +    answer<1> f(int&)       { return answer<1>(); }
> +    answer<2> f(const int&) { return answer<2>(); }
> +    answer<3> f(int&&)      { return answer<3>(); }
> +
> +    void
> +    test()
> +    {
> +      int i = 0;
> +      const int c = 0;
> +      static_assert(decltype(f(i))::value == 1, "");
> +      static_assert(decltype(f(c))::value == 2, "");
> +      static_assert(decltype(f(0))::value == 3, "");
> +    }
> +
> +  }
> +
> +  namespace test_uniform_initialization
> +  {
> +
> +    struct test
> +    {
> +      static const int zero {};
> +      static const int one {1};
> +    };
> +
> +    static_assert(test::zero == 0, "");
> +    static_assert(test::one == 1, "");
> +
> +  }
> +
> +  namespace test_lambdas
> +  {
> +
> +    void
> +    test1()
> +    {
> +      auto lambda1 = [](){};
> +      auto lambda2 = lambda1;
> +      lambda1();
> +      lambda2();
> +    }
> +
> +    int
> +    test2()
> +    {
> +      auto a = [](int i, int j){ return i + j; }(1, 2);
> +      auto b = []() -> int { return '0'; }();
> +      auto c = [=](){ return a + b; }();
> +      auto d = [&](){ return c; }();
> +      auto e = [a, &b](int x) mutable {
> +        const auto identity = [](int y){ return y; };
> +        for (auto i = 0; i < a; ++i)
> +          a += b--;
> +        return x + identity(a + b);
> +      }(0);
> +      return a + b + c + d + e;
> +    }
> +
> +    int
> +    test3()
> +    {
> +      const auto nullary = [](){ return 0; };
> +      const auto unary = [](int x){ return x; };
> +      using nullary_t = decltype(nullary);
> +      using unary_t = decltype(unary);
> +      const auto higher1st = [](nullary_t f){ return f(); };
> +      const auto higher2nd = [unary](nullary_t f1){
> +        return [unary, f1](unary_t f2){ return f2(unary(f1())); };
> +      };
> +      return higher1st(nullary) + higher2nd(nullary)(unary);
> +    }
> +
> +  }
> +
> +  namespace test_variadic_templates
> +  {
> +
> +    template <int...>
> +    struct sum;
> +
> +    template <int N0, int... N1toN>
> +    struct sum<N0, N1toN...>
> +    {
> +      static constexpr auto value = N0 + sum<N1toN...>::value;
> +    };
> +
> +    template <>
> +    struct sum<>
> +    {
> +      static constexpr auto value = 0;
> +    };
> +
> +    static_assert(sum<>::value == 0, "");
> +    static_assert(sum<1>::value == 1, "");
> +    static_assert(sum<23>::value == 23, "");
> +    static_assert(sum<1, 2>::value == 3, "");
> +    static_assert(sum<5, 5, 11>::value == 21, "");
> +    static_assert(sum<2, 3, 5, 7, 11, 13>::value == 41, "");
> +
> +  }
> +
> +  // http://stackoverflow.com/questions/13728184/template-aliases-and-sfinae
> +  // Clang 3.1 fails with headers of libstd++ 4.8.3 when using std::function
> +  // because of this.
> +  namespace test_template_alias_sfinae
> +  {
> +
> +    struct foo {};
> +
> +    template<typename T>
> +    using member = typename T::member_type;
> +
> +    template<typename T>
> +    void func(...) {}
> +
> +    template<typename T>
> +    void func(member<T>*) {}
> +
> +    void test();
> +
> +    void test() { func<foo>(0); }
> +
> +  }
> +
> +}  // namespace cxx11
> +
> +#endif  // __cplusplus >= 201103L
> +
> +]])
> +
> +
> +dnl  Tests for new features in C++14
> +
> +m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_14], [[
> +
> +// If the compiler admits that it is not ready for C++14, why torture it?
> +// Hopefully, this will speed up the test.
> +
> +#ifndef __cplusplus
> +
> +#error "This is not a C++ compiler"
> +
> +#elif __cplusplus < 201402L
> +
> +#error "This is not a C++14 compiler"
> +
> +#else
> +
> +namespace cxx14
> +{
> +
> +  namespace test_polymorphic_lambdas
> +  {
> +
> +    int
> +    test()
> +    {
> +      const auto lambda = [](auto&&... args){
> +        const auto istiny = [](auto x){
> +          return (sizeof(x) == 1UL) ? 1 : 0;
> +        };
> +        const int aretiny[] = { istiny(args)... };
> +        return aretiny[0];
> +      };
> +      return lambda(1, 1L, 1.0f, '1');
> +    }
> +
> +  }
> +
> +  namespace test_binary_literals
> +  {
> +
> +    constexpr auto ivii = 0b0000000000101010;
> +    static_assert(ivii == 42, "wrong value");
> +
> +  }
> +
> +  namespace test_generalized_constexpr
> +  {
> +
> +    template < typename CharT >
> +    constexpr unsigned long
> +    strlen_c(const CharT *const s) noexcept
> +    {
> +      auto length = 0UL;
> +      for (auto p = s; *p; ++p)
> +        ++length;
> +      return length;
> +    }
> +
> +    static_assert(strlen_c("") == 0UL, "");
> +    static_assert(strlen_c("x") == 1UL, "");
> +    static_assert(strlen_c("test") == 4UL, "");
> +    static_assert(strlen_c("another\0test") == 7UL, "");
> +
> +  }
> +
> +  namespace test_lambda_init_capture
> +  {
> +
> +    int
> +    test()
> +    {
> +      auto x = 0;
> +      const auto lambda1 = [a = x](int b){ return a + b; };
> +      const auto lambda2 = [a = lambda1(x)](){ return a; };
> +      return lambda2();
> +    }
> +
> +  }
> +
> +  namespace test_digit_separators
> +  {
> +
> +    constexpr auto ten_million = 100'000'000;
> +    static_assert(ten_million == 100000000, "");
> +
> +  }
> +
> +  namespace test_return_type_deduction
> +  {
> +
> +    auto f(int& x) { return x; }
> +    decltype(auto) g(int& x) { return x; }
> +
> +    template < typename T1, typename T2 >
> +    struct is_same
> +    {
> +      static constexpr auto value = false;
> +    };
> +
> +    template < typename T >
> +    struct is_same<T, T>
> +    {
> +      static constexpr auto value = true;
> +    };
> +
> +    int
> +    test()
> +    {
> +      auto x = 0;
> +      static_assert(is_same<int, decltype(f(x))>::value, "");
> +      static_assert(is_same<int&, decltype(g(x))>::value, "");
> +      return x;
> +    }
> +
> +  }
> +
> +}  // namespace cxx14
> +
> +#endif  // __cplusplus >= 201402L
> +
> +]])
> +
> +
> +dnl  Tests for new features in C++17
> +
> +m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_17], [[
> +
> +// If the compiler admits that it is not ready for C++17, why torture it?
> +// Hopefully, this will speed up the test.
> +
> +#ifndef __cplusplus
> +
> +#error "This is not a C++ compiler"
> +
> +#elif __cplusplus <= 201402L
> +
> +#error "This is not a C++17 compiler"
> +
> +#else
> +
> +#if defined(__clang__)
> +  #define REALLY_CLANG
> +#else
> +  #if defined(__GNUC__)
> +    #define REALLY_GCC
> +  #endif
> +#endif
> +
> +#include <initializer_list>
> +#include <utility>
> +#include <type_traits>
> +
> +namespace cxx17
> +{
> +
> +#if !defined(REALLY_CLANG)
> +  namespace test_constexpr_lambdas
> +  {
> +
> +    // TODO: test it with clang++ from git
> +
> +    constexpr int foo = [](){return 42;}();
> +
> +  }
> +#endif // !defined(REALLY_CLANG)
> +
> +  namespace test::nested_namespace::definitions
> +  {
> +
> +  }
> +
> +  namespace test_fold_expression
> +  {
> +
> +    template<typename... Args>
> +    int multiply(Args... args)
> +    {
> +      return (args * ... * 1);
> +    }
> +
> +    template<typename... Args>
> +    bool all(Args... args)
> +    {
> +      return (args && ...);
> +    }
> +
> +  }
> +
> +  namespace test_extended_static_assert
> +  {
> +
> +    static_assert (true);
> +
> +  }
> +
> +  namespace test_auto_brace_init_list
> +  {
> +
> +    auto foo = {5};
> +    auto bar {5};
> +
> +    static_assert(std::is_same<std::initializer_list<int>, decltype(foo)>::value);
> +    static_assert(std::is_same<int, decltype(bar)>::value);
> +  }
> +
> +  namespace test_typename_in_template_template_parameter
> +  {
> +
> +    template<template<typename> typename X> struct D;
> +
> +  }
> +
> +  namespace test_fallthrough_nodiscard_maybe_unused_attributes
> +  {
> +
> +    int f1()
> +    {
> +      return 42;
> +    }
> +
> +    [[nodiscard]] int f2()
> +    {
> +      [[maybe_unused]] auto unused = f1();
> +
> +      switch (f1())
> +      {
> +      case 17:
> +        f1();
> +        [[fallthrough]];
> +      case 42:
> +        f1();
> +      }
> +      return f1();
> +    }
> +
> +  }
> +
> +  namespace test_extended_aggregate_initialization
> +  {
> +
> +    struct base1
> +    {
> +      int b1, b2 = 42;
> +    };
> +
> +    struct base2
> +    {
> +      base2() {
> +        b3 = 42;
> +      }
> +      int b3;
> +    };
> +
> +    struct derived : base1, base2
> +    {
> +        int d;
> +    };
> +
> +    derived d1 {{1, 2}, {}, 4};  // full initialization
> +    derived d2 {{}, {}, 4};      // value-initialized bases
> +
> +  }
> +
> +  namespace test_general_range_based_for_loop
> +  {
> +
> +    struct iter
> +    {
> +      int i;
> +
> +      int& operator* ()
> +      {
> +        return i;
> +      }
> +
> +      const int& operator* () const
> +      {
> +        return i;
> +      }
> +
> +      iter& operator++()
> +      {
> +        ++i;
> +        return *this;
> +      }
> +    };
> +
> +    struct sentinel
> +    {
> +      int i;
> +    };
> +
> +    bool operator== (const iter& i, const sentinel& s)
> +    {
> +      return i.i == s.i;
> +    }
> +
> +    bool operator!= (const iter& i, const sentinel& s)
> +    {
> +      return !(i == s);
> +    }
> +
> +    struct range
> +    {
> +      iter begin() const
> +      {
> +        return {0};
> +      }
> +
> +      sentinel end() const
> +      {
> +        return {5};
> +      }
> +    };
> +
> +    void f()
> +    {
> +      range r {};
> +
> +      for (auto i : r)
> +      {
> +        [[maybe_unused]] auto v = i;
> +      }
> +    }
> +
> +  }
> +
> +  namespace test_lambda_capture_asterisk_this_by_value
> +  {
> +
> +    struct t
> +    {
> +      int i;
> +      int foo()
> +      {
> +        return [*this]()
> +        {
> +          return i;
> +        }();
> +      }
> +    };
> +
> +  }
> +
> +  namespace test_enum_class_construction
> +  {
> +
> +    enum class byte : unsigned char
> +    {};
> +
> +    byte foo {42};
> +
> +  }
> +
> +  namespace test_constexpr_if
> +  {
> +
> +    template <bool cond>
> +    int f ()
> +    {
> +      if constexpr(cond)
> +      {
> +        return 13;
> +      }
> +      else
> +      {
> +        return 42;
> +      }
> +    }
> +
> +  }
> +
> +  namespace test_selection_statement_with_initializer
> +  {
> +
> +    int f()
> +    {
> +      return 13;
> +    }
> +
> +    int f2()
> +    {
> +      if (auto i = f(); i > 0)
> +      {
> +        return 3;
> +      }
> +
> +      switch (auto i = f(); i + 4)
> +      {
> +      case 17:
> +        return 2;
> +
> +      default:
> +        return 1;
> +      }
> +    }
> +
> +  }
> +
> +#if !defined(REALLY_CLANG)
> +  namespace test_template_argument_deduction_for_class_templates
> +  {
> +
> +    // TODO: test it with clang++ from git
> +
> +    template <typename T1, typename T2>
> +    struct pair
> +    {
> +      pair (T1 p1, T2 p2)
> +        : m1 {p1},
> +          m2 {p2}
> +      {}
> +
> +      T1 m1;
> +      T2 m2;
> +    };
> +
> +    void f()
> +    {
> +      [[maybe_unused]] auto p = pair{13, 42u};
> +    }
> +
> +  }
> +#endif // !defined(REALLY_CLANG)
> +
> +  namespace test_non_type_auto_template_parameters
> +  {
> +
> +    template <auto n>
> +    struct B
> +    {};
> +
> +    B<5> b1;
> +    B<'a'> b2;
> +
> +  }
> +
> +#if !defined(REALLY_CLANG)
> +  namespace test_structured_bindings
> +  {
> +
> +    // TODO: test it with clang++ from git
> +
> +    int arr[2] = { 1, 2 };
> +    std::pair<int, int> pr = { 1, 2 };
> +
> +    auto f1() -> int(&)[2]
> +    {
> +      return arr;
> +    }
> +
> +    auto f2() -> std::pair<int, int>&
> +    {
> +      return pr;
> +    }
> +
> +    struct S
> +    {
> +      int x1 : 2;
> +      volatile double y1;
> +    };
> +
> +    S f3()
> +    {
> +      return {};
> +    }
> +
> +    auto [ x1, y1 ] = f1();
> +    auto& [ xr1, yr1 ] = f1();
> +    auto [ x2, y2 ] = f2();
> +    auto& [ xr2, yr2 ] = f2();
> +    const auto [ x3, y3 ] = f3();
> +
> +  }
> +#endif // !defined(REALLY_CLANG)
> +
> +#if !defined(REALLY_CLANG)
> +  namespace test_exception_spec_type_system
> +  {
> +
> +    // TODO: test it with clang++ from git
> +
> +    struct Good {};
> +    struct Bad {};
> +
> +    void g1() noexcept;
> +    void g2();
> +
> +    template<typename T>
> +    Bad
> +    f(T*, T*);
> +
> +    template<typename T1, typename T2>
> +    Good
> +    f(T1*, T2*);
> +
> +    static_assert (std::is_same_v<Good, decltype(f(g1, g2))>);
> +
> +  }
> +#endif // !defined(REALLY_CLANG)
> +
> +  namespace test_inline_variables
> +  {
> +
> +    template<class T> void f(T)
> +    {}
> +
> +    template<class T> inline T g(T)
> +    {
> +      return T{};
> +    }
> +
> +    template<> inline void f<>(int)
> +    {}
> +
> +    template<> int g<>(int)
> +    {
> +      return 5;
> +    }
> +
> +  }
> +
> +}  // namespace cxx17
> +
> +#endif  // __cplusplus <= 201402L
> +
> +]])
> diff --git a/m4/openvswitch.m4 b/m4/openvswitch.m4
> index 648750ab5ad9..00ffad35f6b0 100644
> --- a/m4/openvswitch.m4
> +++ b/m4/openvswitch.m4
> @@ -620,3 +620,19 @@ AC_DEFUN([OVS_LIBTOOL_VERSIONS],
>    AC_MSG_RESULT([libX-$OVS_MAJOR.$OVS_MINOR.so.$LT_CURRENT.0.$OVS_MICRO)])
>    AC_SUBST(OVS_LTINFO)
>      ])
> +
> +dnl OVS does not use C++ itself, but it provides public header files
> +dnl that a C++ compiler should accept, so when --enable-Werror is in
> +dnl effect and a C++ compiler is available, we enable building a C++
> +dnl source file that #includes all the public headers, as a way to
> +dnl ensure that they are acceptable as C++.
> +AC_DEFUN([OVS_CHECK_CXX],
> +  [AC_REQUIRE([AC_PROG_CXX])
> +   AC_REQUIRE([OVS_ENABLE_WERROR])
> +   AX_CXX_COMPILE_STDCXX([11], [], [optional])
> +   if test $enable_Werror = yes && test $HAVE_CXX11 = 1; then
> +     enable_cxx=:
> +   else
> +     enable_cxx=false
> +   fi
> +   AM_CONDITIONAL([HAVE_CXX], [$enable_cxx])])
> --
> 2.10.2



-- 
Russell Bryant


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