Be more conservative about QPDF_DLL_CLASS with mingw (fixes #799)

* Define it even though previous experiments have shown it to be
  unnecessary since it seems like it may be necessary after all
* Add QPDF_DLL_CLASS to QPDFObjectHelper and QPDFDocumentHelper in
  case there's some future unknown reason why someone may want to have
  them and/or in case it helps with the weird
  QPDFNameTreeObjectHelper problem.

README-maintainer

@@ -179,9 +179,11 @@ CODING RULES
   the shared object boundary (or "shared library boundary" -- we may
   use either term in comments and documentation). In particular,
   anything new derived from Pipeline or InputSource should be marked
-  with QPDF_DLL_CLASS, but we don't need to do it for QPDFObjectHelper
+  with QPDF_DLL_CLASS. We shouldn't need to do it for QPDFObjectHelper
   or QPDFDocumentHelper subclasses since there's no reason to use
-  dynamic_cast with those.
+  dynamic_cast with those, but doing it anyway may help with some
+  strange cases for mingw or with some code generators that may
+  systematically do this for other reasons.
 
   IMPORTANT NOTE ABOUT QPDF_DLL_CLASS: On mingw, the vtable for a
   class with some virtual methods and no pure virtual methods seems

include/qpdf/DLL.h

@@ -42,14 +42,16 @@
 #  define QPDF_DLL
 # endif
 # define QPDF_DLL_PRIVATE
-# define QPDF_DLL_CLASS
 #elif defined __GNUC__
 # define QPDF_DLL __attribute__((visibility("default")))
 # define QPDF_DLL_PRIVATE __attribute__((visibility("hidden")))
-# define QPDF_DLL_CLASS QPDF_DLL
 #else
 # define QPDF_DLL
 # define QPDF_DLL_PRIVATE
+#endif
+#ifdef __GNUC__
+# define QPDF_DLL_CLASS QPDF_DLL
+#else
 # define QPDF_DLL_CLASS
 #endif
 
@@ -87,6 +89,9 @@ for a more in-depth discussion.
     multi-platform and building both static and shared libraries that
     use the same headers, so we don't bother.
 
+  * If we don't export base classes with mingw, the vtables don't end
+    up in the DLL.
+
 * On Linux (and other similar systems):
 
   * Common compilers such as gcc and clang export all symbols into the

include/qpdf/QPDFDocumentHelper.hh

@@ -36,7 +36,7 @@
 // introduced to allow creation of higher level helper functions
 // without polluting the public interface of QPDF.
 
-class QPDFDocumentHelper
+class QPDF_DLL_CLASS QPDFDocumentHelper
 {
   public:
     QPDF_DLL
@@ -45,7 +45,7 @@ class QPDFDocumentHelper
     {
     }
     QPDF_DLL
-    virtual ~QPDFDocumentHelper() = default;
+    virtual ~QPDFDocumentHelper();
     QPDF_DLL
     QPDF&
     getQPDF()

include/qpdf/QPDFObjectHelper.hh

@@ -37,7 +37,7 @@
 // introduced to allow creation of higher level helper functions
 // without polluting the public interface of QPDFObjectHandle.
 
-class QPDFObjectHelper
+class QPDF_DLL_CLASS QPDFObjectHelper
 {
   public:
     QPDF_DLL
@@ -46,7 +46,7 @@ class QPDFObjectHelper
     {
     }
     QPDF_DLL
-    virtual ~QPDFObjectHelper() = default;
+    virtual ~QPDFObjectHelper();
     QPDF_DLL
     QPDFObjectHandle
     getObjectHandle()

libqpdf/CMakeLists.txt

@@ -59,6 +59,7 @@ set(libqpdf_SOURCES
   QPDFAnnotationObjectHelper.cc
   QPDFArgParser.cc
   QPDFCryptoProvider.cc
+  QPDFDocumentHelper.cc
   QPDFEFStreamObjectHelper.cc
   QPDFEmbeddedFileDocumentHelper.cc
   QPDFExc.cc
@@ -74,6 +75,7 @@ set(libqpdf_SOURCES
   QPDFNumberTreeObjectHelper.cc
   QPDFObject.cc
   QPDFObjectHandle.cc
+  QPDFObjectHelper.cc
   QPDFObjGen.cc
   QPDFOutlineDocumentHelper.cc
   QPDFOutlineObjectHelper.cc

libqpdf/QPDFDocumentHelper.cc

@@ -0,0 +1,7 @@
+#include <qpdf/QPDFDocumentHelper.hh>
+
+QPDFDocumentHelper::~QPDFDocumentHelper()
+{
+    // Must be explicit and not inline -- see QPDF_DLL_CLASS in
+    // README-maintainer
+}

libqpdf/QPDFObjectHelper.cc

@@ -0,0 +1,7 @@
+#include <qpdf/QPDFObjectHelper.hh>
+
+QPDFObjectHelper::~QPDFObjectHelper()
+{
+    // Must be explicit and not inline -- see QPDF_DLL_CLASS in
+    // README-maintainer
+}