Default Port values

When defining a port, it might be convenient to add a default value, i.e. the value that the port should have if not specified in the XML.

note

Some of the examples shown in this tutorial will require version 4.5.2 or later.

Default InputPorts

Let's consider a node initializing multiple ports. We use a custom type Point2D, but the same is true for simple types, such as int, double or string.

  static PortsList providedPorts()
  {
    return { 
      BT::InputPort<Point2D>("input"),
      BT::InputPort<Point2D>("pointA", Point2D{1, 2}, "default value is x=1, y=2"),
      BT::InputPort<Point2D>("pointB", "3,4",         "default value is x=3, y=4"),
      BT::InputPort<Point2D>("pointC", "{point}",     "point by default to BB entry {point}"),
      BT::InputPort<Point2D>("pointD", "{=}",         "point by default to BB entry {pointD}") 
    };
  }

The very first one (input) has no default value and it is mandatory to providing either a value or blackboard entry in the XML.

Default values

BT::InputPort<Point2D>("pointA", Point2D{1, 2}, "...");

If the template specialization convertFromString<Point2D>() is implemented, we can use that too.

In other words, the following syntaxes should be equivalent, if our convertFromString expects two comma-separated values:

BT::InputPort<Point2D>("pointB", "3,4", "...");
// should be equivalent to:
BT::InputPort<Point2D>("pointB", Point2D{3, 4}, "...");

Default blackboard entry

Alternatively, we can define the default blackboard entry that the port should point at.

BT::InputPort<Point2D>("pointC", "{point}", "...");

If the name of the port and the blackboard entry are the same, you can use "{=}"

BT::InputPort<Point2D>("pointD", "{=}", "...");
// equivalent to:
BT::InputPort<Point2D>("pointD", "{pointD}", "...");

Default OutputPorts

Output ports are more limited and can only point to a blackboard entry. You can still use "{=}" when the two names are the same.

  static PortsList providedPorts()
  {
    return { 
      BT::OutputPort<Point2D>("result", "{target}", "point by default to BB entry {target}");
    };
  }