Allow Node functionality to use methods with noexcept modifiers.

[scrouthtv]

Feature request

Feature description

The rclcpp::Node methods

• create_subscription()
• create_wall_timer()
• create_service()
• add_on_set_parameters_callback(), remove_on_set_parameters_callback(), set_on_parameters_set_callback()
  all take a callback as parameter. I think, the callback methods should be able to be marked as noexcept. For example, this currently does not work:

#include <chrono>
#include <functional>
#include <memory>

#include <rclcpp/rclcpp.hpp>

class DemoNode : public rclcpp::Node {
 public:
  DemoNode() : rclcpp::Node("demo127") {
    this->create_wall_timer(
      std::chrono::milliseconds(100),
      std::bind(&DemoNode::read, this));
    }
	
 private:
  void read() noexcept {
    // etc.
  }
};

int main(int argc, char ** argv) {
  rclcpp::init(argc, argv);
  auto node = std::make_shared<DemoNode>();
  rclcpp::spin(node);
  return rclcpp::shutdown();
}

Won't compile with this error:

error: no matching function for call to ‘DemoNode::create_wall_timer(std::chrono::milliseconds, std::_Bind_helper<false, void (DemoNode::*)() noexcept, DemoNode*>::type)

Removing the noexcept modifier fixes the issue.

I think it should be possible to add the noexcept modifier to callbacks in timers, services, etc.

Going further, it may only make sense to have these callbacks forced to be noexcept?

Implementation considerations

noexcept has only been available since C++11.

[cottsay]

Hi there - thanks for filing the ticket.

As an immediate path forward, is there any reason you couldn't use a lambda to wrap your noexcept function?

[clalancette]

Going further, it may only make sense to have these callbacks forced to be noexcept?

I don't think we should force the callbacks to be noexcept; there are legitimate reasons for throwing exceptions in them.

That said, I do agree that it would be nice to accept methods that have the noexcept attribute on them. We probably won't have time to look into it right now, but if you'd like to contribute a pull request to add it we'd be happy to review it.