Rework conversion to std::error_category to not allocate (closes #78)

include/boost/system/detail/error_category.hpp

@@ -48,6 +48,11 @@ class std_category;
 #pragma GCC diagnostic ignored "-Wnon-virtual-dtor"
 #endif
 
+#if defined(BOOST_MSVC)
+#pragma warning(push)
+#pragma warning(disable: 4351) //  new behavior: elements of array will be default initialized
+#endif
+
 class BOOST_SYMBOL_VISIBLE error_category
 {
 private:
@@ -76,13 +81,21 @@ class BOOST_SYMBOL_VISIBLE error_category
 
     boost::ulong_long_type id_;
 
+    static std::size_t const stdcat_size_ = 4 * sizeof( void const* );
+
+    union
+    {
+        mutable unsigned char stdcat_[ stdcat_size_ ];
+        void const* stdcat_align_;
+    };
+
 #if defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
 
-    mutable std::atomic< boost::system::detail::std_category* > ps_;
+    mutable std::atomic< unsigned > sc_init_;
 
 #else
 
-    boost::system::detail::std_category* ps_;
+    unsigned sc_init_;
 
 #endif
 
@@ -103,11 +116,11 @@ class BOOST_SYMBOL_VISIBLE error_category
 
 #endif
 
-    BOOST_SYSTEM_CONSTEXPR error_category() BOOST_NOEXCEPT: id_( 0 ), ps_()
+    BOOST_SYSTEM_CONSTEXPR error_category() BOOST_NOEXCEPT: id_( 0 ), stdcat_(), sc_init_()
     {
     }
 
-    explicit BOOST_SYSTEM_CONSTEXPR error_category( boost::ulong_long_type id ) BOOST_NOEXCEPT: id_( id ), ps_()
+    explicit BOOST_SYSTEM_CONSTEXPR error_category( boost::ulong_long_type id ) BOOST_NOEXCEPT: id_( id ), stdcat_(), sc_init_()
     {
     }
 
@@ -158,14 +171,22 @@ class BOOST_SYMBOL_VISIBLE error_category
     }
 
 #if defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
+
+    void init_stdcat() const;
+
 # if defined(__SUNPRO_CC) // trailing __global is not supported
     operator std::error_category const & () const;
 # else
     operator std::error_category const & () const BOOST_SYMBOL_VISIBLE;
 # endif
+
 #endif
 };
 
+#if defined(BOOST_MSVC)
+#pragma warning(pop)
+#endif
+
 #if ( defined( BOOST_GCC ) && BOOST_GCC >= 40600 ) || defined( BOOST_CLANG )
 #pragma GCC diagnostic pop
 #endif

include/boost/system/detail/error_category_impl.hpp

@@ -98,25 +98,60 @@ inline char const * error_category::message( int ev, char * buffer, std::size_t
 #if defined(BOOST_SYSTEM_HAS_SYSTEM_ERROR)
 
 #include <boost/system/detail/std_category_impl.hpp>
+#include <mutex>
+#include <new>
 
 namespace boost
 {
 namespace system
 {
 
+namespace detail
+{
+
+template<class = void> struct stdcat_mx_holder
+{
+    static std::mutex mx_;
+};
+
+template<class T> std::mutex stdcat_mx_holder<T>::mx_;
+
+} // namespace detail
+
+inline BOOST_NOINLINE void error_category::init_stdcat() const
+{
+    static_assert( sizeof( stdcat_ ) >= sizeof( boost::system::detail::std_category ), "sizeof(stdcat_) is not enough for std_category" );
+
+#if defined(BOOST_MSVC) && BOOST_MSVC < 1900
+    // no alignof
+#else
+
+    static_assert( alignof( decltype(stdcat_align_) ) >= alignof( boost::system::detail::std_category ), "alignof(stdcat_) is not enough for std_category" );
+
+#endif
+
+    std::lock_guard<std::mutex> lk( boost::system::detail::stdcat_mx_holder<>::mx_ );
+
+    if( sc_init_.load( std::memory_order_acquire ) == 0 )
+    {
+        ::new( static_cast<void*>( stdcat_ ) ) boost::system::detail::std_category( this, 0 );
+        sc_init_.store( 1, std::memory_order_release );
+    }
+}
+
 inline error_category::operator std::error_category const & () const
 {
     if( id_ == detail::generic_category_id )
     {
 // This condition must be the same as the one in error_condition.hpp
 #if defined(BOOST_SYSTEM_AVOID_STD_GENERIC_CATEGORY)
 
-    static const boost::system::detail::std_category generic_instance( this, 0x1F4D3 );
-    return generic_instance;
+        static const boost::system::detail::std_category generic_instance( this, 0x1F4D3 );
+        return generic_instance;
 
 #else
 
-    return std::generic_category();
+        return std::generic_category();
 
 #endif
     }
@@ -126,47 +161,22 @@ inline error_category::operator std::error_category const & () const
 // This condition must be the same as the one in error_code.hpp
 #if defined(BOOST_SYSTEM_AVOID_STD_SYSTEM_CATEGORY)
 
-    static const boost::system::detail::std_category system_instance( this, 0x1F4D7 );
-    return system_instance;
+        static const boost::system::detail::std_category system_instance( this, 0x1F4D7 );
+        return system_instance;
 
 #else
 
-    return std::system_category();
+        return std::system_category();
 
 #endif
     }
 
-    detail::std_category* p = ps_.load( std::memory_order_acquire );
-
-    if( p != 0 )
+    if( sc_init_.load( std::memory_order_acquire ) == 0 )
     {
-        return *p;
+        init_stdcat();
     }
 
-    // One `std_category` object is allocated for every
-    // user-defined `error_category` that is converted to
-    // `std::error_category`.
-    //
-    // This one-time allocation will show up on leak checkers.
-    // That's unavoidable. There is no way to deallocate the
-    // `std_category` object because first, `error_category`
-    // is a literal type (so it can't have a destructor) and
-    // second, `error_category` needs to be usable during program
-    // shutdown.
-    //
-    // https://github.com/boostorg/system/issues/78
-
-    detail::std_category* q = new detail::std_category( this, 0 );
-
-    if( ps_.compare_exchange_strong( p, q, std::memory_order_release, std::memory_order_acquire ) )
-    {
-        return *q;
-    }
-    else
-    {
-        delete q;
-        return *p;
-    }
+    return *reinterpret_cast<boost::system::detail::std_category const*>( stdcat_ );
 }
 
 } // namespace system