FindProtobuf: add optional export declaration macro to generated cpp files

Help/release/dev/FindProtobuf-export-macro.rst

+FindProtobuf-export-macro
+-------------------------
+
+* The :module:`FindProtobuf` module :command:`protobuf_generate_cpp`
+  command gained an ``EXPORT_MACRO`` option to specify the name of
+  a DLL export markup macro.

Modules/FindProtobuf.cmake

@@ -76,6 +76,7 @@
 #   include_directories(${Protobuf_INCLUDE_DIRS})
 #   include_directories(${CMAKE_CURRENT_BINARY_DIR})
 #   protobuf_generate_cpp(PROTO_SRCS PROTO_HDRS foo.proto)
+#   protobuf_generate_cpp(PROTO_SRCS PROTO_HDRS EXPORT_MACRO DLL_EXPORT foo.proto)
 #   protobuf_generate_python(PROTO_PY foo.proto)
 #   add_executable(bar bar.cc ${PROTO_SRCS} ${PROTO_HDRS})
 #   target_link_libraries(bar ${Protobuf_LIBRARIES})
@@ -89,12 +90,15 @@
 #
 #   Add custom commands to process ``.proto`` files to C++::
 #
-#     protobuf_generate_cpp (<SRCS> <HDRS> [<ARGN>...])
+#     protobuf_generate_cpp (<SRCS> <HDRS> [EXPORT_MACRO <MACRO>] [<ARGN>...])
 #
 #   ``SRCS``
 #     Variable to define with autogenerated source files
 #   ``HDRS``
 #     Variable to define with autogenerated header files
+#   ``EXPORT_MACRO``
+#     is a macro which should expand to ``__declspec(dllexport)`` or
+#     ``__declspec(dllimport)`` depending on what is being compiled.
 #   ``ARGN``
 #     ``.proto`` files
 #
@@ -110,14 +114,21 @@
 #     ``.proto`` filess
 
 function(PROTOBUF_GENERATE_CPP SRCS HDRS)
-  if(NOT ARGN)
+  cmake_parse_arguments(protobuf "" "EXPORT_MACRO" "" ${ARGN})
+
+  set(PROTO_FILES "${protobuf_UNPARSED_ARGUMENTS}")
+  if(NOT PROTO_FILES)
     message(SEND_ERROR "Error: PROTOBUF_GENERATE_CPP() called without any proto files")
     return()
   endif()
 
+  if(protobuf_EXPORT_MACRO)
+    set(DLL_EXPORT_DECL "dllexport_decl=${protobuf_EXPORT_MACRO}:")
+  endif()
+
   if(PROTOBUF_GENERATE_CPP_APPEND_PATH)
     # Create an include path for each file specified
-    foreach(FIL ${ARGN})
+    foreach(FIL ${PROTO_FILES})
       get_filename_component(ABS_FIL ${FIL} ABSOLUTE)
       get_filename_component(ABS_PATH ${ABS_FIL} PATH)
       list(FIND _protobuf_include_path ${ABS_PATH} _contains_already)
@@ -145,7 +156,7 @@ function(PROTOBUF_GENERATE_CPP SRCS HDRS)
 
   set(${SRCS})
   set(${HDRS})
-  foreach(FIL ${ARGN})
+  foreach(FIL ${PROTO_FILES})
     get_filename_component(ABS_FIL ${FIL} ABSOLUTE)
     get_filename_component(FIL_WE ${FIL} NAME_WE)
     if(NOT PROTOBUF_GENERATE_CPP_APPEND_PATH)
@@ -162,7 +173,7 @@ function(PROTOBUF_GENERATE_CPP SRCS HDRS)
       OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/${FIL_WE}.pb.cc"
              "${CMAKE_CURRENT_BINARY_DIR}/${FIL_WE}.pb.h"
       COMMAND  ${Protobuf_PROTOC_EXECUTABLE}
-      ARGS --cpp_out  ${CMAKE_CURRENT_BINARY_DIR} ${_protobuf_include_path} ${ABS_FIL}
+      ARGS "--cpp_out=${DLL_EXPORT_DECL}${CMAKE_CURRENT_BINARY_DIR}" ${_protobuf_include_path} ${ABS_FIL}
       DEPENDS ${ABS_FIL} ${Protobuf_PROTOC_EXECUTABLE}
       COMMENT "Running C++ protocol buffer compiler on ${FIL}"
       VERBATIM )