Fix Windows: Shared (DLL) Builds Export

GPUDataRegistry in src/elements/mixin/dynamicdata.H uses
function-local statics (next_id, registry) inside template member
functions. With explicit template instantiation
IMPACTX_GPUDATA_INSTANTIATE), this works
for static builds. However, for DLL/shared library builds, each DLL
that links the library could get its own copy of these statics unless
the explicit instantiations are properly exported with visibility
attributes.

Author: ax3l

CMakeLists.txt

@@ -175,6 +175,15 @@ set_target_properties(lib PROPERTIES
     WINDOWS_EXPORT_ALL_SYMBOLS ON
 )
 
+include(GenerateExportHeader)
+generate_export_header(lib
+    BASE_NAME impactx
+    EXPORT_FILE_NAME src/impactx_export.H
+)
+if(NOT BUILD_SHARED_LIBS)
+    target_compile_definitions(lib PUBLIC IMPACTX_STATIC_DEFINE)
+endif()
+
 # Optional: build only a single TU
 if(ImpactX_UNITY_BUILD)
     set_target_properties(lib PROPERTIES

src/elements/mixin/dynamicdata.H

@@ -10,6 +10,8 @@
 #ifndef IMPACTX_ELEMENTS_MIXIN_DYNAMICDATA_H
 #define IMPACTX_ELEMENTS_MIXIN_DYNAMICDATA_H
 
+#include "impactx_export.H"
+
 #include <AMReX_Gpu.H>
 
 #include <map>
@@ -137,7 +139,7 @@ namespace impactx::elements::mixin
  * @param ElementType The element class (must define `using DynamicData = ...`)
  */
 #define IMPACTX_GPUDATA_INSTANTIATE(ElementType) \
-    template struct impactx::elements::mixin::GPUDataRegistry< \
+    template struct IMPACTX_EXPORT impactx::elements::mixin::GPUDataRegistry< \
         ElementType::DynamicData::DataType>;
 
 #endif // IMPACTX_ELEMENTS_MIXIN_DYNAMICDATA_H