behavior_tree_lite

A header-only behavior tree library for C++23. You compose the tree with &&, || and !, and the compiler turns it into one nested type. Nothing is dispatched through a virtual call and nothing goes on the heap, except the one node built for that (DynamicAction).

auto tree = (CheckBattery{} && Inspect{}) || ReturnHome{};
tree.process(Tick{}, ctx);

Why it exists

Most behavior tree libraries build the tree at runtime: nodes on the heap, virtual tick(), often an XML loader in front. That buys runtime editing and visual tools. It costs an allocation per node and an indirect call per tick, and a typo in the tree shows up when the robot runs, not when it compiles.

If the tree is known when you build, none of that is needed. Here the tree is a type. A node with the wrong event type or a missing process() is a compile error, and the optimizer sees through the whole tree.

	behavior_tree_lite	BehaviorTree.CPP
Dispatch	Templates, inlined	Virtual calls
Tree definition	C++ operators: && \|\| !	XML or a runtime builder
Tree shape	Fixed at compile time	Loaded and changed at runtime
Allocations	None outside DynamicAction	Heap-allocated nodes
Header-only	Yes	No (shared library)
C++ standard	C++23	C++17
Good fit	Hot loops, embedded, game AI	Large trees, visual editors, logging

Pick BehaviorTree.CPP if you need to change the tree without rebuilding, or want Groot. Pick this one if the tree ships with the binary.

Where to go next

• Getting started: install, then a three-node tree.
• DSL: the operators, flattening, and how event and context types are found.
• Nodes: every composite, decorator and leaf, and exactly what each returns.
• Blackboard: a typed context with no runtime lookups.
• Events and debugging: std::variant events and print_tree.
• ROS 2 example and PX4 SITL example.
• Performance: what the benchmark measures, and what it doesn't.