warning C4141: 'dllexport': used more than once

[tsondergaard@…]

The following warnings are emitted by Visual Studio 2017 when using boost/serialization as a shared library (and defining BOOST_ALL_DYN_LINK when using the library):

C:\...\boost\include\boost-1_65_1\boost/archive/codecvt_null.hpp(68): warning C4141: 'dllexport': used more than once
C:\...\boost\include\boost-1_65_1\boost/archive/codecvt_null.hpp(78): warning C4141: 'dllexport': used more than once
C:\...\boost\include\boost-1_65_1\boost/serialization/singleton.hpp(94): warning C4141: 'dllexport': used more than once

The problem is use of BOOST_*_DECL and BOOST_DLLEXPORT at the same time. Removing the BOOST_DLLEXPORT macro from those lines as done in the following patch fixes the problem.

diff --git a/boost/include/boost-1_65_1/boost/archive/codecvt_null.hpp b/boost/include/boost-1_65_1/boost/archive/codecvt_null.hpp
index 7bce2b9b..332ae9f9 100644
--- a/boost/include/boost-1_65_1/boost/archive/codecvt_null.hpp
+++ b/boost/include/boost-1_65_1/boost/archive/codecvt_null.hpp
@@ -65,7 +65,7 @@ public:
 template<>
 class BOOST_SYMBOL_VISIBLE codecvt_null<wchar_t> : public std::codecvt<wchar_t, char, std::mbstate_t>
 {
-    virtual BOOST_WARCHIVE_DECL BOOST_DLLEXPORT std::codecvt_base::result
+    virtual BOOST_WARCHIVE_DECL std::codecvt_base::result
     do_out(
         std::mbstate_t & state,
         const wchar_t * first1,
@@ -75,7 +75,7 @@ class BOOST_SYMBOL_VISIBLE codecvt_null<wchar_t> : public std::codecvt<wchar_t,
         char * last2,
         char * & next2
     ) const BOOST_USED;
-    virtual BOOST_WARCHIVE_DECL BOOST_DLLEXPORT std::codecvt_base::result
+    virtual BOOST_WARCHIVE_DECL std::codecvt_base::result
     do_in(
         std::mbstate_t & state,
         const char * first1,
diff --git a/boost/include/boost-1_65_1/boost/serialization/singleton.hpp b/boost/include/boost-1_65_1/boost/serialization/singleton.hpp
index b50afedb..b9e470fa 100644
--- a/boost/include/boost-1_65_1/boost/serialization/singleton.hpp
+++ b/boost/include/boost-1_65_1/boost/serialization/singleton.hpp
@@ -91,7 +91,7 @@ class BOOST_SYMBOL_VISIBLE singleton_module :
     public boost::noncopyable
 {
 private:
-    BOOST_SERIALIZATION_DECL BOOST_DLLEXPORT static bool & get_lock() BOOST_USED;
+    BOOST_SERIALIZATION_DECL static bool & get_lock() BOOST_USED;
 public:
     BOOST_DLLEXPORT static void lock(){
         get_lock() = true;

[Conrad Poelman <cpboost@…>]

We have seen the same problem. We applied the changes to remove BOOST_DLLEXPORT from the three places and it eliminated the compile warnings, but left us with link warnings about importing exported symbols. (Sorry, I had saved the full text of the warnings but Trac tossed away all my text when I went to add an attachment.)

Instead we found if we removed BOOST_SERIALIZATION_DECL / BOOST_WARCHIVE_DECL from the same lines, and left in BOOST_DLLEXPORT, both compile and link warnings went away.

Also we removed them from lines 21 & 49 of codecvt_null.cpp to eliminate two more warnings that occur while compiling Boost itself.