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Version: 4.5

Main Concepts

BehaviorTree.CPP is a C++ library that can be easily integrated into your favourite distributed middleware, such as ROS or SmartSoft.

You can statically link it into your application (for example a game).

These are the main concepts which you need to understand first.

Nodes vs Trees​

The user must create his/her own ActionNodes and ConditionNodes (LeafNodes); this library helps you to compose them easily into trees.

Think about the LeafNodes as the building blocks which you need to compose a complex system. If Nodes were lego bricks, your trees would be lego sets.

By definition, your custom Nodes are (or should be) highly reusable.

Instantiate trees at run-time using the XML format​

Despite the fact that the library is written in C++, trees themselves can be created and composed at run-time, more specifically, at deployment-time, using an XML base scripting language.

An XML format is described in details here, but the best way to learn the syntax is following the tutorials.

The tick() callbacks​

Any TreeNode can be seen as a mechanism to invoke a callback, i.e. to run a piece of code. What this callback does is up to you.

In most of the following tutorials, our Actions will simply print messages on console or sleep for a certain amount of time to simulate a long calculation.

In production code, especially in Model Driven Development and Component Based Software Engineering, an Action/Condition would probably communicate to other components or services of the system.

// The simplest callback you can wrap into a BT Action
NodeStatus HelloTick()
{
  std::cout << "Hello World\n"; 
  return NodeStatus::SUCCESS;
}

// Allow the library to create Actions that invoke HelloTick()
// (explained in the tutorials)
factory.registerSimpleAction("Hello", std::bind(HelloTick));
tip

The factory may create multiple instances of the node Hello.

Create custom nodes with inheritance​

In the example above, a specific type of TreeNodes which invoke HelloTick was created using a function pointer (dependency injection).

Generally, to define a custom TreeNode, you should inherit from the class TreeNode or, more specifically, its derived classes:

  • ActionNodeBase
  • ConditionNode
  • DecoratorNode

As a reference please look at the first tutorial.

Dataflow, Ports and Blackboard​

Ports are explained in detail in the second and third tutorials.

For the time being, it is important to know that:

  • A Blackboard is a key/value storage shared by all the Nodes of a Tree.

  • Ports are a mechanism that Nodes can use to exchange information between each other.

  • Ports are "connected" using the same key of the blackboard.

  • The number, name and kind of ports of a Node must be known at compilation-time (C++); connections between ports are done at deployment-time (XML).

  • You can store as value any C++ type (we use a _type erasure technique similar to std::any).