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Version: 4.3

Remapping ports of a SubTrees

In the CrossDoor example, we saw that a SubTree looks like a single leaf Node from the point of view of its parent tree.

To avoid name clashing in very large trees, any tree and subtree use a different instance of the Blackboard.

For this reason, we need to explicitly connect the ports of a tree to those of its subtrees.

You won't need to modify your C++ implementation since this remapping is done entirely in the XML definition.


Let's consider this Behavior Tree.


<root BTCPP_format="4">

<BehaviorTree ID="MainTree">
<Script script=" move_goal='1;2;3' " />
<SubTree ID="MoveRobot" target="{move_goal}"
result="{move_result}" />
<SaySomething message="{move_result}"/>

<BehaviorTree ID="MoveRobot">
<MoveBase goal="{target}"/>
<Script script=" result:='goal reached' " />
<Script script=" result:='error' " />


You may notice that:

  • We have a MainTree that includes a subtree called MoveRobot.
  • We want to "connect" (i.e. "remap") ports inside the MoveRobot subtree with other ports in the MainTree.
  • This is done with the syntax used in the example above.

The CPP code

Not much to be done here. We use the debugMessage method to inspect the value of the blackboard.

int main()
BT::BehaviorTreeFactory factory;


auto tree = factory.createTree("MainTree");

// Keep ticking until the end

// let's visualize some information about the current state of the blackboards.
std::cout << "\n------ First BB ------" << std::endl;
std::cout << "\n------ Second BB------" << std::endl;

return 0;

/* Expected output:

------ First BB ------
move_result (std::string)
move_goal (Pose2D)

------ Second BB------
[result] remapped to port of parent tree [move_result]
[target] remapped to port of parent tree [move_goal]