Add constexpr to virtual functions on C++20 or later. Refs #141.

Author: pdimov

include/boost/system/detail/config.hpp

@@ -38,6 +38,18 @@
 # define BOOST_SYSTEM_CONSTEXPR
 #endif
 
+// BOOST_SYSTEM_HAS_CXX20_CONSTEXPR
+
+#if defined(__cpp_constexpr) && __cpp_constexpr >= 201907L
+# define BOOST_SYSTEM_HAS_CXX20_CONSTEXPR
+#endif
+
+#if defined(BOOST_SYSTEM_HAS_CXX20_CONSTEXPR)
+# define BOOST_SYSTEM_CXX20_CONSTEXPR constexpr
+#else
+# define BOOST_SYSTEM_CXX20_CONSTEXPR
+#endif
+
 // BOOST_SYSTEM_DEPRECATED
 
 #if defined(__clang__)

include/boost/system/detail/error_category.hpp

@@ -94,31 +94,31 @@ class BOOST_SYMBOL_VISIBLE error_category
 
 public:
 
-    virtual const char * name() const noexcept = 0;
+    virtual const char* name() const noexcept = 0;
 
-    virtual error_condition default_error_condition( int ev ) const noexcept;
-    virtual bool equivalent( int code, const error_condition & condition ) const noexcept;
-    virtual bool equivalent( const error_code & code, int condition ) const noexcept;
+    BOOST_SYSTEM_CXX20_CONSTEXPR virtual error_condition default_error_condition( int ev ) const noexcept;
+    BOOST_SYSTEM_CXX20_CONSTEXPR virtual bool equivalent( int code, error_condition const& condition ) const noexcept;
+    BOOST_SYSTEM_CXX20_CONSTEXPR virtual bool equivalent( error_code const& code, int condition ) const noexcept;
 
     virtual std::string message( int ev ) const = 0;
-    virtual char const * message( int ev, char * buffer, std::size_t len ) const noexcept;
+    virtual char const* message( int ev, char* buffer, std::size_t len ) const noexcept;
 
-    virtual bool failed( int ev ) const noexcept
+    BOOST_SYSTEM_CXX20_CONSTEXPR virtual bool failed( int ev ) const noexcept
     {
         return ev != 0;
     }
 
-    friend BOOST_SYSTEM_CONSTEXPR bool operator==( error_category const & lhs, error_category const & rhs ) noexcept
+    friend BOOST_SYSTEM_CONSTEXPR bool operator==( error_category const& lhs, error_category const& rhs ) noexcept
     {
         return rhs.id_ == 0? &lhs == &rhs: lhs.id_ == rhs.id_;
     }
 
-    friend BOOST_SYSTEM_CONSTEXPR bool operator!=( error_category const & lhs, error_category const & rhs ) noexcept
+    friend BOOST_SYSTEM_CONSTEXPR bool operator!=( error_category const& lhs, error_category const& rhs ) noexcept
     {
         return !( lhs == rhs );
     }
 
-    friend BOOST_SYSTEM_CONSTEXPR bool operator<( error_category const & lhs, error_category const & rhs ) noexcept
+    friend BOOST_SYSTEM_CONSTEXPR bool operator<( error_category const& lhs, error_category const& rhs ) noexcept
     {
         if( lhs.id_ < rhs.id_ )
         {
@@ -135,15 +135,15 @@ class BOOST_SYMBOL_VISIBLE error_category
             return false; // equal
         }
 
-        return std::less<error_category const *>()( &lhs, &rhs );
+        return std::less<error_category const*>()( &lhs, &rhs );
     }
 
     void init_stdcat() const;
 
 # if defined(__SUNPRO_CC) // trailing __global is not supported
-    operator std::error_category const & () const;
+    operator std::error_category const& () const;
 # else
-    operator std::error_category const & () const BOOST_SYMBOL_VISIBLE;
+    operator std::error_category const& () const BOOST_SYMBOL_VISIBLE;
 # endif
 };
 
@@ -162,7 +162,7 @@ static const boost::ulong_long_type generic_category_id = ( boost::ulong_long_ty
 static const boost::ulong_long_type system_category_id = generic_category_id + 1;
 static const boost::ulong_long_type interop_category_id = generic_category_id + 2;
 
-BOOST_SYSTEM_CONSTEXPR inline bool failed_impl( int ev, error_category const & cat )
+BOOST_SYSTEM_CONSTEXPR inline bool failed_impl( int ev, error_category const& cat )
 {
     if( cat.id_ == system_category_id || cat.id_ == generic_category_id )
     {

include/boost/system/detail/error_category_impl.hpp

@@ -26,22 +26,22 @@ namespace system
 
 // error_category default implementation
 
-inline error_condition error_category::default_error_condition( int ev ) const noexcept
+BOOST_SYSTEM_CXX20_CONSTEXPR inline error_condition error_category::default_error_condition( int ev ) const noexcept
 {
     return error_condition( ev, *this );
 }
 
-inline bool error_category::equivalent( int code, const error_condition & condition ) const noexcept
+BOOST_SYSTEM_CXX20_CONSTEXPR inline bool error_category::equivalent( int code, error_condition const& condition ) const noexcept
 {
     return default_error_condition( code ) == condition;
 }
 
-inline bool error_category::equivalent( const error_code & code, int condition ) const noexcept
+BOOST_SYSTEM_CXX20_CONSTEXPR inline bool error_category::equivalent( error_code const& code, int condition ) const noexcept
 {
     return code.equals( condition, *this );
 }
 
-inline char const * error_category::message( int ev, char * buffer, std::size_t len ) const noexcept
+inline char const* error_category::message( int ev, char* buffer, std::size_t len ) const noexcept
 {
     if( len == 0 )
     {
@@ -116,7 +116,7 @@ inline void error_category::init_stdcat() const
 #pragma GCC diagnostic ignored "-Wstrict-aliasing"
 #endif
 
-inline BOOST_NOINLINE error_category::operator std::error_category const & () const
+inline BOOST_NOINLINE error_category::operator std::error_category const& () const
 {
     if( id_ == detail::generic_category_id )
     {

test/CMakeLists.txt

@@ -135,6 +135,8 @@ boost_test(TYPE run SOURCES ec_hash_value_test.cpp)
 
 boost_test(TYPE run SOURCES std_interop_test16.cpp)
 
+boost_test(TYPE run SOURCES failed_constexpr_test2.cpp)
+
 # result
 
 boost_test(TYPE run SOURCES result_default_construct.cpp)

test/Jamfile.v2

@@ -168,6 +168,8 @@ run ec_hash_value_test.cpp ;
 
 run std_interop_test16.cpp ;
 
+run failed_constexpr_test2.cpp ;
+
 # result
 
 run result_default_construct.cpp ;

test/failed_constexpr_test2.cpp

@@ -0,0 +1,54 @@
+// Copyright 2026 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/system/error_code.hpp>
+#include <boost/system/error_condition.hpp>
+#include <boost/config/pragma_message.hpp>
+#include <boost/static_assert.hpp>
+#include <cstdio>
+
+#if !defined(BOOST_SYSTEM_HAS_CXX20_CONSTEXPR)
+
+BOOST_PRAGMA_MESSAGE("Skipping constexpr test, BOOST_SYSTEM_HAS_CXX20_CONSTEXPR isn't defined")
+int main() {}
+
+#else
+
+namespace sys = boost::system;
+
+class user_category: public sys::error_category
+{
+public:
+
+    constexpr virtual const char* name() const noexcept
+    {
+        return "user";
+    }
+
+    virtual std::string message( int ev ) const
+    {
+        char buffer[ 256 ];
+        std::snprintf( buffer, sizeof( buffer ), "user message %d", ev );
+
+        return buffer;
+    }
+};
+
+static constexpr user_category s_user_cat;
+
+constexpr sys::error_code ec( 1, s_user_cat );
+
+BOOST_STATIC_ASSERT( ec.failed() );
+BOOST_STATIC_ASSERT( ec );
+BOOST_STATIC_ASSERT( !!ec );
+
+constexpr sys::error_condition en( 1, s_user_cat );
+
+BOOST_STATIC_ASSERT( en.failed() );
+BOOST_STATIC_ASSERT( en );
+BOOST_STATIC_ASSERT( !!en );
+
+int main() {}
+
+#endif