improve dynamic loading example

Author: jll63

doc/modules/ROOT/examples/CMakeLists.txt

@@ -17,7 +17,7 @@ endif()
 
 file(GLOB cpp_files "*.cpp")
 
-foreach(cpp ${cpp_files})
+foreach (cpp ${cpp_files})
     cmake_path(REMOVE_EXTENSION cpp LAST_ONLY OUTPUT_VARIABLE stem)
     string(REGEX REPLACE ".*/" "" stem ${stem})
     add_executable(${stem} ${cpp})
@@ -27,4 +27,7 @@ foreach(cpp ${cpp_files})
 endforeach()
 
 add_subdirectory(headers_namespaces)
-add_subdirectory(shared_libs)
+
+if (NOT WIN32)
+  add_subdirectory(shared_libs)
+endif()

doc/modules/ROOT/examples/shared_libs/CMakeLists.txt

@@ -3,7 +3,9 @@
 # See accompanying file LICENSE_1_0.txt
 # or copy at http://www.boost.org/LICENSE_1_0.txt)
 
-message(STATUS "Building dynamic_shared loading examples")
+message(STATUS "Building shared library examples")
+
+add_compile_definitions(BOOST_OPENMETHOD_ENABLE_RUNTIME_CHECKS)
 
 # ------------------------------------------------------------------------------
 # static linking
@@ -23,7 +25,9 @@ add_executable(dynamic dynamic_main.cpp)
 set_target_properties(dynamic PROPERTIES ENABLE_EXPORTS ON)
 target_link_libraries(dynamic Boost::openmethod Boost::dll)
 add_dependencies(dynamic shared)
-add_test(NAME dynamic_shared COMMAND dynamic)
+if (NOT WIN32)
+    add_test(NAME dynamic_shared COMMAND dynamic)
+endif()
 
 # ------------------------------------------------------------------------------
 # dynamic loading, indirect virtual_ptrs
@@ -40,4 +44,6 @@ target_compile_definitions(
 set_target_properties(indirect PROPERTIES ENABLE_EXPORTS ON)
 target_link_libraries(indirect Boost::openmethod Boost::dll)
 add_dependencies(indirect indirect_shared)
-add_test(NAME indirect_shared COMMAND indirect)
+if (NOT WIN32)
+  add_test(NAME indirect_shared COMMAND indirect)
+endif()

doc/modules/ROOT/pages/shared_libraries.adoc

@@ -77,6 +77,39 @@ overriders provided by the shared library are removed from the method.
 include::{shared}/dynamic_main.cpp[tag=unload]
 ----
 
+### Windows
+
+If we try this example on Windows, the result is disappointing:
+
+```
+Before loading the shared library.
+cow meets wolf -> greet
+wolf meets cow -> greet
+
+After loading the shared library.
+cow meets wolf -> greet
+wolf meets cow -> greet
+cow meets tiger -> unknown class struct Tiger
+```
+
+What happens here is that the program and the DLL have their own copies of
+"global" variables. When the DLL is loaded, its static constructors run, and
+they add overriders to _their_ copy of the method (the `method::fn` static
+variable for the given name and signature). They are ignored when the main
+program calls `initialize`.
+
+Likewise, `BOOST_OPENMETHOD_CLASSES(Tiger, Carnivore);` in the DLL adds `Tiger`
+to the DLL's copy of the registry. For the perspective of the program's
+registry, the class does not exist.
+
+In theory, this can be fixed by adding `__declspec(dllimport)` and
+`__declspec(dllexport)` attributes where needed. However, this is not practical,
+because programs and DLLs can both import and export registries and methods. The
+underlying objects are instantiated from templates, which complicates the
+matter. Research is being done on this subject. However, as of now, dynamic
+loading is supported on Windows only if it does not attempt to share a registry
+across modules.
+
 ### Indirect Vptrs
 
 `initialize` rebuilds the v-tables in the registry. This invalidates all the
@@ -87,10 +120,10 @@ their execution.
 
 Otherwise, indirect v-table pointers must be used. This is achieved by using a
 registry that contains the cpp:indirect_vptr[] policy.
-`<boost/openmethod/default_registry.hpp>` provides an `indirect_registry` which
-has the same policies as `default_registry`, plus `indirect_vptr`. We can use it
-to override the default registry, for example using a compiler command-line
-switch (`-DBOOST_OPENMETHOD_DEFAULT_REGISTRY=indirect_registry`).
+`<boost/openmethod/default_registry.hpp>` provides an cpp:indirect_registry[]
+that has the same policies as `default_registry`, plus `indirect_vptr`. We can
+use it to override the default registry, for example using a compiler
+command-line switch (`-DBOOST_OPENMETHOD_DEFAULT_REGISTRY=indirect_registry`).
 
 Here is an example of a program that carries `virtual_ptr`{empty}'s across
 `initialize` calls: