Difficult for callers to satisfy prereqs of `MultibodyPlant::GetXByName`

[ggould-tri]

https://app.slack.com/client/T0JNB2DS4/C2WBPQDB7/thread/C2WBPQDB7-1585062720.104100 (mentioning @jwnimmer-tri here since he's on that thread)

• GetJointByName is templated on joint type and is documented to throw on the wrong type.
• It can be called with Joint and the result dynamic cast to the desired type, but using dynamic cast for control flow is GSG-discouraged.
• HasJointNamed is not templated and so cannot tell me if a joint has the right type.
• Joint implements type_name, but it is not static and so there is no way to get from a subclass template argument to the type name to check without knowing how to construct that subclass.

The underlying issue is that MBP assumes that its callers already knew everything about the tree -- of course a caller knows what type that joint is, they created it! But code that has to run over a number of different SDFs (which might have joints of the same name but different type) really does not have that information at compile-time.

The result is recurring, un-factorable (because the string and type name covary undetectably at compile-time) code like this:

  if (plant->HasJointNamed("dishwasher_lower_rack_spring", id) &&
      (plant->GetJointByName("dishwasher_lower_rack_spring", id).type_name ==
      "prismatic") &&
      plant->HasJointNamed("dishwasher_lower_rack_joint", id) &&
      (plant->GetJointByName("dishwasher_lower_rack_joint", id).type_name ==
       "prismatic")) {
    const PrismaticJoint<double>* lower_rack_vertical_joint =
        plant->GetJointByName<PrismaticJoint>(
            "dishwasher_lower_rack_spring", id);
    const PrismaticJoint<double>* lower_rack_horizontal_joint =
        plant->GetJointByName<PrismaticJoint>(
            "dishwasher_lower_rack_joint", id);

Note that each of those if conditions is required by the exceptions clause of the GetJointByName documentation. Note in particular that it is impossible for a user to write the function template <JointType> JointType* GetJointByNameOrNull(name, id).

I can see a number of ways to fix this, including:

• Make GetXByName return optional or null or similar
• Make HasXNamed templated on the type and return false if the joint has a different type
• Make type_name static so that you do not need to know how to instantiate a template argument to get its type name

[sherm1]

@ggould-tri I couldn't follow all of that -- how could type_name() be static? Can you sketch what you think would be an ideal version of the code you wrote above?

cc @amcastro-tri

[ggould-tri]

The ideal version wouid be something like

const PrismaticJoint<double>* lower_rack_vertical_joint =
        plant->GetJointByNameOrNull<PrismaticJoint>(
            "dishwasher_lower_rack_spring", id);
const PrismaticJoint<double>* lower_rack_horizontal_joint =
        plant->GetJointByNameOrNull<PrismaticJoint>(
            "dishwasher_lower_rack_joint", id);
if (lower_rack_vertical_joint && lower_rack_horizontal_joint) {

However that imaginary GetJointByNameOrNull is currently impossible to write correctly under GSG.

If there were a static type name available then I could write GetJointByNameOrNull<J>() by comparing J::type_name() to GetJointByName<Joint>().type_name(). But as it is to get type_name from a template parameter I have to be able to construct an object of that type, and there is no common ctor signature for all Joints so that's impossible.

[jwnimmer-tri]

FWIW I think all joints have Joint::kTypeName constant. (Not that we should use it.)

[amcastro-tri]

And Joint::kTypeName is public, so that'd allow us to write the GSG compliant GetJointByNameOrNull()?

[ggould-tri]

https://drake.mit.edu/doxygen_cxx/classdrake_1_1multibody_1_1_joint.html does not show kTypeName. kTypeName does not appear in joint.h or its superclass header. If a static type name constant is meant to be part of the joint concept, I think we need to say so somewhere?

[jwnimmer-tri]

As I read it, the only call to action here is to fix Doxygen to tell users that kTypeName exists, possibly near GetJointByName.