add samples/windll: minimal dllvar mechanism example

Shows the minimal pattern for sharing a registry state across an exe/DLL boundary on Windows using the dllvar policy:
- shared.hpp: defines BOOST_OPENMETHOD_EXPORT/IMPORT_DEFAULT_REGISTRY based on WINDLL_OWNER, declares Animal/Dog and a speak() method
- main.cpp: defines WINDLL_OWNER (dllexport), loads the plugin at runtime before initialize(), asserts cross-module dispatch works
- plugin.cpp: no WINDLL_OWNER (dllimport), adds the Dog-specific overrider via static constructor
- CMakeLists.txt: no circular dependency - the exe does not link against the plugin import lib; LoadLibraryA loads the DLL before initialize()

Non-Windows: both TUs compiled into a single executable.

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

Author: jll63

CMakeLists.txt

@@ -220,4 +220,9 @@ if (BOOST_OPENMETHOD_BUILD_TESTS)
     if (BOOST_OPENMETHOD_BUILD_EXAMPLES)
         add_subdirectory(doc/modules/ROOT/examples)
     endif ()
+
+    # Samples
+    if (BOOST_OPENMETHOD_BUILD_EXAMPLES)
+        add_subdirectory(samples/windll)
+    endif ()
 endif ()

samples/windll/CMakeLists.txt

@@ -0,0 +1,38 @@
+# Copyright (c) 2018-2025 Jean-Louis Leroy
+# Distributed under the Boost Software License, Version 1.0.
+# See accompanying file LICENSE_1_0.txt
+# or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+# Illustrates the dllvar mechanism: the exe owns (exports) the registry state;
+# the DLL imports it, contributing overriders that are consolidated by
+# initialize() before first use.
+#
+# On non-Windows both compilation units are linked into a single executable.
+
+if (WIN32 OR CYGWIN)
+    # Exe exports the registry state and loads the plugin at runtime.
+    add_executable(windll_main main.cpp)
+    target_compile_definitions(windll_main PRIVATE WINDLL_OWNER)
+    set_target_properties(windll_main PROPERTIES ENABLE_EXPORTS ON)
+    target_link_libraries(windll_main PRIVATE Boost::openmethod)
+
+    # DLL imports the registry state from the exe.
+    add_library(windll_plugin SHARED plugin.cpp)
+    target_link_libraries(windll_plugin PRIVATE Boost::openmethod windll_main)
+    set_target_properties(windll_plugin PROPERTIES ENABLE_EXPORTS ON)
+
+    # windll_main loads windll_plugin at runtime; build both before testing.
+    add_custom_target(windll_all ALL DEPENDS windll_main windll_plugin)
+
+    add_test(
+        NAME windll
+        COMMAND windll_main
+        WORKING_DIRECTORY $<TARGET_FILE_DIR:windll_main>)
+else()
+    # Non-Windows: single executable, no DLL decoration needed.
+    add_executable(windll_main main.cpp plugin.cpp)
+    target_compile_definitions(windll_main PRIVATE WINDLL_OWNER)
+    target_link_libraries(windll_main PRIVATE Boost::openmethod)
+
+    add_test(NAME windll COMMAND windll_main)
+endif()

samples/windll/main.cpp

@@ -0,0 +1,33 @@
+// Copyright (c) 2018-2025 Jean-Louis Leroy
+// Distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE_1_0.txt
+// or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+// This module owns the registry state (WINDLL_OWNER -> dllexport).
+#define WINDLL_OWNER
+#include "shared.hpp"
+#include <boost/openmethod/initialize.hpp>
+#include <cassert>
+#include <string>
+#ifdef _WIN32
+#include <windows.h>
+#endif
+
+using namespace boost::openmethod;
+
+BOOST_OPENMETHOD_CLASSES(Animal, Dog);
+BOOST_OPENMETHOD_OVERRIDE(speak, (virtual_ptr<Animal>), const char*) { return "?"; }
+
+int main() {
+#ifdef _WIN32
+    // Load plugin before initialize() so its static constructors register
+    // the Dog overrider into the shared registry state.
+    LoadLibraryA("windll_plugin.dll");
+#endif
+    initialize();
+
+    Dog d;
+    assert(speak(virtual_ptr<Dog>::final(d)) == std::string("woof"));
+    Animal a;
+    assert(speak(virtual_ptr<Animal>::final(a)) == std::string("?"));
+}

samples/windll/plugin.cpp

@@ -0,0 +1,12 @@
+// Copyright (c) 2018-2025 Jean-Louis Leroy
+// Distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE_1_0.txt
+// or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+// This module imports the registry state from the exe (no WINDLL_OWNER).
+#include "shared.hpp"
+
+using namespace boost::openmethod;
+
+BOOST_OPENMETHOD_CLASSES(Animal, Dog);
+BOOST_OPENMETHOD_OVERRIDE(speak, (virtual_ptr<Dog>), const char*) { return "woof"; }

samples/windll/shared.hpp

@@ -0,0 +1,24 @@
+// Copyright (c) 2018-2025 Jean-Louis Leroy
+// Distributed under the Boost Software License, Version 1.0.
+// See accompanying file LICENSE_1_0.txt
+// or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#pragma once
+
+// Define WINDLL_OWNER in the module that owns (exports) the registry state.
+// All other modules that link against it automatically import it.
+#ifdef _WIN32
+# ifdef WINDLL_OWNER
+#   define BOOST_OPENMETHOD_EXPORT_DEFAULT_REGISTRY
+# else
+#   define BOOST_OPENMETHOD_IMPORT_DEFAULT_REGISTRY
+# endif
+#endif
+
+#include <boost/openmethod.hpp>
+
+struct Animal { virtual ~Animal() = default; };
+struct Dog    : Animal {};
+
+BOOST_OPENMETHOD_CLASSES(Animal, Dog);
+BOOST_OPENMETHOD(speak, (boost::openmethod::virtual_ptr<Animal>), const char*);