export for the event type support function

[iuhilnehc-ynos]

There is no reason to ignore event_message.

Please see

rosidl/rosidl_generator_c/resource/srv__type_support.h.em

Line 19 in a63f32e

package_name=package_name, message=service.event_message,

[iuhilnehc-ynos]

rosidl_runtime_cpp only provides the template function to get the message type support (also including the service and action)

rosidl/rosidl_runtime_cpp/include/rosidl_typesupport_cpp/message_type_support.hpp

Lines 24 to 25 in 034e526

	template<typename T>
	const rosidl_message_type_support_t * get_message_type_support_handle();

which is different from rosidl_runtime_c using a macro to call the function

rosidl/rosidl_runtime_c/include/rosidl_runtime_c/message_type_support_struct.h

Lines 98 to 100 in 7583b95

	#define ROSIDL_GET_MSG_TYPE_SUPPORT(PkgName, MsgSubfolder, MsgName) \
	ROSIDL_TYPESUPPORT_INTERFACE__MESSAGE_SYMBOL_NAME( \
	rosidl_typesupport_c, PkgName, MsgSubfolder, MsgName)()

, I wonder if it's necessary to add the declaration for each message/service in the header file by rosidl_generator_cpp.

• e.g., build/example_interfaces/rosidl_generator_cpp/example_interfaces/srv/detail/add_two_ints__type_support.hpp

#include "rosidl_typesupport_cpp/message_type_support.hpp"

#ifdef __cplusplus
extern "C"
{
#endif
// Forward declare the get type support functions for this type.
ROSIDL_GENERATOR_CPP_PUBLIC_example_interfaces
const rosidl_message_type_support_t *
  ROSIDL_TYPESUPPORT_INTERFACE__MESSAGE_SYMBOL_NAME(
  rosidl_typesupport_cpp,
  example_interfaces,
  srv,
  AddTwoInts_Request
)();
#ifdef __cplusplus
}
#endif

// already included above
// #include "rosidl_typesupport_cpp/message_type_support.hpp"

#ifdef __cplusplus
extern "C"
{
#endif
// Forward declare the get type support functions for this type.
ROSIDL_GENERATOR_CPP_PUBLIC_example_interfaces
const rosidl_message_type_support_t *
  ROSIDL_TYPESUPPORT_INTERFACE__MESSAGE_SYMBOL_NAME(
  rosidl_typesupport_cpp,
  example_interfaces,
  srv,
  AddTwoInts_Response
)();
#ifdef __cplusplus
}
#endif

As we know, the definition of messages
https://github.com/ros2/rosidl_typesupport/blob/9062d91c09a79c3b9902a8f06bca4e994121cc52/rosidl_typesupport_cpp/resource/srv__type_support.cpp.em#L3-L24
can be used for loading dynamically without the header file,

https://github.com/ros2/rclcpp/blob/95adde2a19426c23d6ff6e13fab21d62e5e2ae69/rclcpp/src/rclcpp/typesupport_helpers.cpp#L124-L129

    std::string symbol_name = typesupport_identifier + "__get_message_type_support_handle__" +
      package_name + "__" + (middle_module.empty() ? "msg" : middle_module) + "__" + type_name;

    const rosidl_message_type_support_t * (* get_ts)() = nullptr;
    // This will throw runtime_error if the symbol was not found.
    get_ts = reinterpret_cast<decltype(get_ts)>(library.get_symbol(symbol_name));

My question is,
do we need to add the declaration in the header file by rosidl_generator_cpp, or remove the declaration because it seems unnecessary and unuseful?

NOTE: examples of getting message type support

[iuhilnehc-ynos]

I think it's not good to add rosidl_typesupport_interface
e.g.,

#define ROSIDL_GET_MSG_TYPE_SUPPORT_CPP(PkgName, MsgSubfolder, MsgName) \
  ROSIDL_TYPESUPPORT_INTERFACE__MESSAGE_SYMBOL_NAME( \
    rosidl_typesupport_cpp, PkgName, MsgSubfolder, MsgName)()

in rosidl_runtime_cpp, but it seems it's the follow-up PR of #703.

[fujitatomoya]

I think this can be added along with response and request message types.

[fujitatomoya]

can be used for loading dynamically without the header file,
do we need to add the declaration in the header file by rosidl_generator_cpp, or remove #703 because it seems unnecessary and unuseful?

it is doable, but the difference is between compile time and runtime? i might be mistaken.

[iuhilnehc-ynos]

it is doable, but the difference is between compile time and runtime?

Yes, but no.

Let me take a generated service typesupport in cpp as an example,

• build/example_interfaces/rosidl_typesupport_cpp/example_interfaces/srv/add_two_ints__type_support.cpp

namespace rosidl_typesupport_cpp
{

template<>
ROSIDL_TYPESUPPORT_CPP_PUBLIC
const rosidl_service_type_support_t *
get_service_type_support_handle<example_interfaces::srv::AddTwoInts>()
{
  return &::example_interfaces::srv::rosidl_typesupport_cpp::AddTwoInts_service_type_support_handle;
}

}  // namespace rosidl_typesupport_cpp

#ifdef __cplusplus
extern "C"
{
#endif

ROSIDL_TYPESUPPORT_CPP_PUBLIC
const rosidl_service_type_support_t *
ROSIDL_TYPESUPPORT_INTERFACE__SERVICE_SYMBOL_NAME(rosidl_typesupport_cpp, example_interfaces, srv, AddTwoInts)() {
  return ::rosidl_typesupport_cpp::get_service_type_support_handle<example_interfaces::srv::AddTwoInts>();
}

#ifdef __cplusplus
}
#endif

Is there anybody want to use ROSIDL_TYPESUPPORT_INTERFACE__SERVICE_SYMBOL_NAME(rosidl_typesupport_cpp,...) instead of get_service_type_support_handle<>?

Please notice:

1. C++ template is also built in the compile time
2. the ROSIDL_TYPESUPPORT_INTERFACE__SERVICE_SYMBOL_NAME(rosidl_typesupport_cpp,...) is calling the template inside.
3. use in runtime (load dynamically) doesn't need the declaration in the header file

This is why I was confused that adding the declaration ROSIDL_TYPESUPPORT_INTERFACE__SERVICE_SYMBOL_NAME(rosidl_typesupport_cpp,...) in the header file?

[fujitatomoya]

@iuhilnehc-ynos thanks for the explanation.

Is there anybody want to use ROSIDL_TYPESUPPORT_INTERFACE__SERVICE_SYMBOL_NAME(rosidl_typesupport_cpp,...) instead of get_service_type_support_handle<>?

in short, i think this is unnecessary.

according to your explanation, it seems that we do not need to have these macros in the header file, although i am not familiar with history, there could be intention to add. lets hear opinion from maintainers.

CC: @adityapande-1995 @methylDragon @quarkytale