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Version: 4.6


About this library

This C++ library provides a framework to create BehaviorTrees. It is designed to be flexible, easy to use and fast.

Even if our main use-case is robotics, you can use this library to build AI for games, or to replace Finite State Machines in your application.

BehaviorTree.CPP has many interesting features, when compared to other implementations:

  • It makes asynchronous Actions, i.e. non-blocking routines, a first-class citizen.
  • Trees are created at run-time, using an interpreted language (based on XML).
  • It includes a logging/profiling infrastructure that allows the user to visualize, record, replay and analyze state transitions.
  • You can link statically your custom TreeNodes or convert them into plugins which are loaded at run-time.

What is a Behavior Tree?

A Behavior Tree (BT) is a way to structure the switching between different tasks in an autonomous agent, such as a robot or a virtual entity in a computer game.

BTs are a very efficient way of creating complex systems that are both modular and reactive. These properties are crucial in many applications, which has led to the spread of BT from computer game programming to many branches of AI and Robotics.

If you are already familiar with Finite State Machines (FSM), you will easily grasp most of the concepts but, hopefully, you will find that BTs are more expressive and easier to reason about.

Think about the Nodes of the tree as a set of building blocks. These blocks are implemented in C++ and are "composable": in other words, they can be "assembled" to build behaviors.

In the image above, you can see as we are arranging these actions in a simple Sequence; actions will be executed in order from left to right. To learn more, visit the page Introduction to BTs.

Main Advantages of Behavior Trees

  • They are intrinsically hierarchical: we can compose complex behaviors, including entire trees as sub-branches of a bigger tree. For instance, the behavior "Fetch Beer" may reuse the tree "Grasp Object".

  • Their graphical representation has a semantic meaning: it is easier to "read" a BT and understand the corresponding workflow. State transitions in FSMs, by comparisons, are harder to understand both in their textual and graphical representation.

  • They are more expressive: Ready to use ControlNodes and DecoratorNodes make it possible to express more complex control flows. The user can extend the "vocabulary" with his/her own custom nodes.

"Ok, but WHY do we need BehaviorTrees (or FSM)?"

Many software systems, robotics being a notable example, are inherently complex.

The usual approach to manage complexity, heterogeneity and scalability is to use the concept of Component Based Software Engineering.

Any existing middleware for robotics took this approach either informally or formally, being ROS, YARP and SmartSoft some notable examples.

A "good" software architecture should have the following characteristics:

  • Modularity.
  • Reusability of components.
  • Composability.
  • Good separation of concerns.

If we don't keep these concepts in mind from the very beginning, we create software that is tightly coupled and less reusable.

Frequently, the business logic of a software system is "spread" into many components and it is hard for the developer to reason about it and to debug errors.

To achieve a strong separation of concerns, it is better to centralize the business logic in a single location.

Finite State Machines were created specifically with this goal in mind, but in recent years, Behavior Trees gained popularity, especially in the game industry.