samples/windll: illustrate the dllvar technique with a plain int

shared.hpp shows the core trick: answer_dllvar has the same type name in every TU, but its base class varies (dllexport_tag in the owner, dllimport_tag in clients). A base class does not enter the mangled name, so static_answer<answer_dllvar, void>::value is ONE shared linker symbol while SFINAE selects the dllexport or dllimport specialization per TU.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: jll63

samples/windll/main.cpp

@@ -3,21 +3,21 @@
 // See accompanying file LICENSE_1_0.txt
 // or copy at http://www.boost.org/LICENSE_1_0.txt)
 
+// Owner: answer_dllvar derives from dllexport_tag here.
 #define WINDLL_OWNER
 #include "shared.hpp"
 #include <cassert>
 #ifdef _WIN32
 #include <windows.h>
 #endif
 
-int answer = 42;
-
 int main() {
+    answer() = 42;
 #ifdef _WIN32
     auto plugin = LoadLibraryA("windll_plugin.dll");
     assert(plugin);
-    auto get_answer = (int (*)())GetProcAddress(plugin, "get_answer");
-    assert(get_answer);
+    auto get_answer = reinterpret_cast<int(*)()>(
+        GetProcAddress(plugin, "get_answer"));
     assert(get_answer() == 42);
     FreeLibrary(plugin);
 #else

samples/windll/plugin.cpp

@@ -3,6 +3,7 @@
 // See accompanying file LICENSE_1_0.txt
 // or copy at http://www.boost.org/LICENSE_1_0.txt)
 
+// Client: answer_dllvar derives from dllimport_tag here (no WINDLL_OWNER).
 #include "shared.hpp"
 
 #ifdef _WIN32
@@ -11,6 +12,6 @@
 #define EXPORT
 #endif
 
-extern "C" EXPORT int get_answer() {
-    return answer;
-}
+// answer() returns a reference to the same static_answer<answer_dllvar, void>::value
+// that main.cpp owns — one shared linker symbol, bound via dllimport.
+extern "C" EXPORT int get_answer() { return answer(); }

samples/windll/shared.hpp

@@ -4,17 +4,56 @@
 // or copy at http://www.boost.org/LICENSE_1_0.txt)
 
 #pragma once
+#include <type_traits>
 
-// The module that owns (exports) the variable defines WINDLL_OWNER before
-// including this header. All other modules import it.
+// Base classes used as SFINAE tags.
+struct dllexport_tag {};
+struct dllimport_tag {};
+
+// answer_dllvar has the same *type name* in every translation unit, but its
+// *base class* varies: dllexport_tag in the owner, dllimport_tag in clients.
+// A base class does not appear in a class's mangled name, so
+// static_answer<answer_dllvar>::value resolves to the SAME linker symbol in
+// all modules — only the export/import attribute differs.
 #ifdef _WIN32
 # ifdef WINDLL_OWNER
-#   define WINDLL_API __declspec(dllexport)
+struct answer_dllvar : dllexport_tag {};
 # else
-#   define WINDLL_API __declspec(dllimport)
+struct answer_dllvar : dllimport_tag {};
 # endif
 #else
-# define WINDLL_API
+struct answer_dllvar {};  // no decoration needed on ELF platforms
+#endif
+
+// Primary template: plain static variable (non-Windows and catch-all).
+template<class Tag, typename = void>
+struct static_answer {
+    static int value;
+};
+template<class Tag, typename Enable>
+int static_answer<Tag, Enable>::value;
+
+#ifdef _WIN32
+// Specialization for dllexport: selected when Tag derives from dllexport_tag.
+// enable_if_t<true> evaluates to void, so the instantiation is
+// static_answer<answer_dllvar, void> — same mangled name as the dllimport case.
+template<class Tag>
+struct static_answer<Tag,
+    std::enable_if_t<std::is_base_of_v<dllexport_tag, Tag>>> {
+    static __declspec(dllexport) int value;
+};
+template<class Tag>
+int static_answer<Tag,
+    std::enable_if_t<std::is_base_of_v<dllexport_tag, Tag>>>::value;
+
+// Specialization for dllimport: no definition — the symbol lives in the owner.
+template<class Tag>
+struct static_answer<Tag,
+    std::enable_if_t<std::is_base_of_v<dllimport_tag, Tag>>> {
+    static __declspec(dllimport) int value;
+};
 #endif
 
-WINDLL_API extern int answer;
+// Accessor: always refers to static_answer<answer_dllvar, void>::value —
+// the one shared linker symbol — regardless of which specialization is active.
+inline int& answer() { return static_answer<answer_dllvar>::value; }