no std::bind with custom arguments for callbacks

[Karsten1987]

I want to pass custom arguments to my callback function and use std::bind for it. However, I believe that function_traits are not allowing passing a functor with custom arguments.

This little demo shows the block which does not compile.
https://github.com/ros2/examples/blob/callback_error/rclcpp_examples/src/services/add_two_ints_server.cpp#L58-L68

[Karsten1987]

@esteve can you provide some help here? I guess that https://github.com/ros2/rclcpp/blob/master/rclcpp/include/rclcpp/function_traits.hpp doesn't allow additional arguments?

[esteve]

@Karsten1987 sure, I just submitted #278 which should work with your example, it's just a quick hack. There's one caveat, though, you can't wrap the std::bind in a std::function (https://github.com/ros2/examples/blob/callback_error/rclcpp_examples/src/services/add_two_ints_server.cpp#L61), you have to pass the std::bind to create_service as is, like this:

auto fcn2 = std::bind(handle_add_two_ints2, std::placeholders::_1, std::placeholders::_2, std::placeholders::_3, i);
node->create_service<example_interfaces::srv::AddTwoInts>("add_two_ints", fcn2);

I haven't tested the pull request heavily, so I can't guarantee that it will work with all cases, but it should work with yours :-)

[esteve]

@Karsten1987 I think this would work, without changing anything in rclcpp:

std::function<void(const std::shared_ptr<rmw_request_id_t>,
  const std::shared_ptr<example_interfaces::srv::AddTwoInts::Request>,
  std::shared_ptr<example_interfaces::srv::AddTwoInts::Response>)> fcn2;
fcn2 = std::bind(handle_add_two_ints2, std::placeholders::_1, std::placeholders::_2, std::placeholders::_3, i);
node->create_service<example_interfaces::srv::AddTwoInts>("add_two_ints", fcn2);

note that I removed the fourth argument in the std::function definition. std::bind can be wrapped in a std::function even though their parameters don't match, as long as when you call it the call signature is compatible. When the wrapped std::bind gets called, it'll be compatible since it only has three placeholders, and i is already known at the time the std::bind was constructed.

[Karsten1987]

@esteve thanks a bunch for iterating on this. Your solution(s) definitely do the trick.

The fact that you can't easily store them in a std::function may become a bit inconvenient at one point; In case you want to store functors in a callback group. Despite the fact that you can't use auto that easily. Is there anyway around this caveat?

In any case, that works just fine and we can simply document this behavior in order to avoid any confusion.

[esteve]

@Karsten1987 you can store std::bind in a std::function without #278, as long as:

• the std::bind uses at most 3 placeholder arguments
• the placeholder arguments appear sequentially. That is, std::bind(func, std::placeholder::_2, std::placeholder::_1) won't work

so given that, this should be fine with the current codebase without any changes to function_traits.hpp:

int i = 42;
std::function<void(const std::shared_ptr<rmw_request_id_t>,
  const std::shared_ptr<example_interfaces::srv::AddTwoInts::Request>,
  std::shared_ptr<example_interfaces::srv::AddTwoInts::Response>)> fcn2 = std::bind(
    handle_add_two_ints2, std::placeholders::_1, std::placeholders::_2,
    std::placeholders::_3, i);
node->create_service<example_interfaces::srv::AddTwoInts>("add_two_ints", fcn2);

[codebot]

In interest of getting Beta 1 frozen today, this is deferred to Beta 2.

[Karsten1987]

@esteve thanks a bunch for the work you've put into it.
As we currently don't have a real show-stopper with it, i'll go ahead and close it. We can open or refer to it when we actually start dealing with MSVC2017.
Cheers!