Unable to use custom allocator with LoanedMessage

[thomasmoore-torc]

Bug report

Required Info:

• Operating System:
  • Linux
• Installation type:
  • From source
• Version or commit hash:
  • iron, jazzy, rolling
• DDS implementation:
  • Fast-RTPS
• Client library (if applicable):
  • rclcpp

Steps to reproduce issue

The LoanedMessage class is defined with an AllocatorT template parameter which is used as the type for the message_allocator_ member variable:

template<typename MessageT, typename AllocatorT = std::allocator<void>>
class LoanedMessage
{
  using MessageAllocatorTraits = rclcpp::allocator::AllocRebind<MessageT, AllocatorT>;
  using MessageAllocator = typename MessageAllocatorTraits::allocator_type;

  MessageAllocator message_allocator_;
};

However, the constructor accepts an allocator parameter which is of type std::allocator<MessageT>:

  LoanedMessage(
    const rclcpp::PublisherBase & pub,
    MessageAllocator allocator)
  : pub_(pub),
    message_(nullptr),
    message_allocator_(std::move(allocator))
  {
  }

This prevents creating instances of LoanedMessage when using a custom allocator which is not based on std::allocator. Our recommendation to resolve this issue is to modify the constructor to use MessageAllocator as the type for the allocator parameter:

   LoanedMessage(
     const rclcpp::PublisherBase & pub,
-    std::allocator<MessageT> allocator)
+    MessageAllocator allocator)
   : pub_(pub),
     message_(nullptr),
     message_allocator_(std::move(allocator))
   {
   }

[fujitatomoya]

@thomasmoore-torc as always, thanks for sending us a patch and good description 👍 i created https://github.com/ros2/rclcpp/pull/2672, if you can review, that would be really appreciated.