Add non-default constructors to avoid initializing messages with dummy data

[alsora]

Feature request

ROS 2 C++ messages are implemented as data-structures with a default constructor.
Usually the workflow is

auto my_header = std_msgs::msg::Header();
my_header.frame_id = "hello";

This results in first zero-initializing the messages and then populating them again with the true data.
This has a non-negligible overhead in case of messages with large bounded members.

We propose to modify the IDL code to include a constructor that takes argument values, for example in the form of a brace-enclosed list

auto my_header = std_msgs::msg::Header({.frame_id = "hello"});

[clalancette]

@alsora We think that the syntax there isn't supported until C++17 or C++20. Can you check which version supports it?

[clalancette]

Looking at it further, this looks to be a C++20 feature: https://en.cppreference.com/w/cpp/language/aggregate_initialization .

Also, it is possible that there don't need to be changes to the core to do this, and it would Just Work(tm) if we upgraded to C++20.

[MiguelCompany]

I think that passing this constant to the constructor and then assign all the fields in the message will serve the same purposes.

[alsora]

@mauropasse did you test the SKIP mode?

[mauropasse]

Yes, passing rosidl_runtime_cpp::MessageInitialization::SKIP as argument to the message constructor has the effect of not initialize anything. So for example, the cost of instantiating an 8MB array message is the same as instantiating a message containing just a bool.

[MiguelCompany]

It might be nice to have an overload of the LoanedMessage constructor that allows forwarding arguments (i.e. rosidl_runtime_cpp::MessageInitialization) to the message constructor when it is called here, and perhaps also passing a similar argument to this rmw function to indicate whether the borrowed memory shall be initialized or not.

[ros-discourse]

This issue has been mentioned on ROS Discourse. There might be relevant details there:

https://discourse.ros.org/t/rosidl-message-builder-utilize-default-field-values/36745/1