Declare_parameter in Dashing

[jdlangs]

@"clalancette

I will point out that there is a difference between get_parameter and declare_parameter in the case (I think) you are describing. In a world where you only have get_parameter, then you either get the value or you don’t, but the parameter is not “visible” on the node in the latter scenario. That is, something like ros2 param list doesn’t show anything, and there is no external indication that a parameter is available to be set. Using declare_parameter, on the other hand, ensures that the parameter is always “visible”, even if it’s value is NOT_SET. I find that to be a highly desirable property of parameters.

I agree this is a nice property. But I don’t see how the new API is relevant to this. If at any point during execution I can declare a new parameter name, how is it different from making the name visible when first getting the parameter value? You can say the best practice is to declare all the names when the process starts but then you might as well say the best practice is to get all the values when it starts. And it’s almost certainly not going to be followed anyway because there will be libraries in other packages that will want to access parameters from the current node and will end up just declaring right before they access.