MessagePoolMemoryStrategy may not compile

Bug report

Required Info:

• Operating System:
  • Ubuntu 22.04 (kernel: 5.15.0-1015-realtime)
• Installation type:
  • from source (humble)
• Version or commit hash:
  • humble
• DDS implementation:
  • n/a
• Client library (if applicable):
  • rclcpp (GCC 11.2.0)

Steps to reproduce issue

Try to refer to the rclcpp::strategies::message_pool_memory_strategy::MessagePoolMemoryStrategy<MessageT, Size, > type with template argument MessageT being a type with non-fixed size.
For instance, define a message type in Bytes.msg:

byte[] data

Then use this type for the memory strategy:

rclcpp::strategies::message_pool_memory_strategy::MessagePoolMemoryStrategy<my_package::msg::Bytes, some_size>

Expected behavior

The code should compile.

Actual behavior

Compilation fails with error:

error: no type named ‘type’ in ‘struct std::enable_if<false, void>’

Additional information

When checking the class definition of MessagePoolMemoryStrategy the issue becomes apparent:

template<
  typename MessageT,
  size_t Size,
  typename std::enable_if<
    rosidl_generator_traits::has_fixed_size<MessageT>::value
  >::type * = nullptr
>
class MessagePoolMemoryStrategy
  : public message_memory_strategy::MessageMemoryStrategy<MessageT>
{
...

The issue is caused by the way how template< bool B, class T = void > struct enable_if is used here.
The resulting struct only has a member type if B is true. Otherwise, no member type is generated and as result, accessing ::type is invalid.
So whenever rosidl_generator_traits::has_fixed_size<MessageT>::value evaluates to false, compilation of will fail, since the code always tries to access the type, no matter what.

A fix might look like this (not yet tested!):

template<
  typename MessageT,
  size_t Size,
  std::enable_if<
    rosidl_generator_traits::has_fixed_size<MessageT>::value,
    void*
  > = nullptr
>
class MessagePoolMemoryStrategy
  : public message_memory_strategy::MessageMemoryStrategy<MessageT>
{
...

Ideally the message pool should be scaled in a way to satisfy an upper bound for the size of individual messages.
Unfortunately, I don't see how this can be achieved with the current API.

Disclaimer

I am not an experienced ROS(2) user. I just tried to implement some benchmarks in a real-time environment. Maybe the short answer to this issue is "RTFM", but I could barely find more documentation than the real-time examples provided with ROS2 and the according tutorials in the docs and since the code causing the compilation error is in fact flawed, I decided to submit this as an issue.

Reproduce the failure with a variable-size Bytes.msg message and rclcpp::strategies::message_pool_memory_strategy::MessagePoolMemoryStrategy. Inspect the class definition and the std::enable_if template argument, then verify that the chosen change allows the example to compile while considering the unresolved upper-bound sizing concern.