📦 box2cpp

C++ bindings for box2d 3.x physics engine (aka box2c)

• Provides classes for all box2d entities: World, Body, Shape, Chain, all joint kinds, DynamicTree.

  • With RAII - automatically destroy box2d objects in destructors

  • All related functions as methods (with const-correctness).

  • Callbacks accept arbitrary lambdas (no need to mess with void * and function pointers).

• Automatically generated, so no missing methods.

• Minimal - we don't wrap enums, most structs (including as b2Vec2 and b2AABB), functions that are not related to a specific class.

And some extras:

• A debug renderer using ImGui.

^{Note: box2d 3.0 is experimental (but seems to work well already), and so are those bindings. They may change as box2d API changes, or when I find new better ways of doing things.}

Hello world

#include <box2cpp/box2c.hpp>

int main()
{
    b2::World w(b2::World::Params{});

    b2::Body::Params bp;
    bp.type = b2_dynamicBody;

    b2::Body b = w.CreateBody(b2::OwningHandle, bp);

    b.CreateShape(
        b2::DestroyWithParent,
        b2::Shape::Params{},
        b2Circle{.center = b2Vec2(), .radius = 3}
    );

    for (int i = 0; i < 10; i++)
    {
        w.Step(1/60.f, 4);
        std::cout << b.GetPosition().y << "\n";
    }

    // No cleanup needed, everything is destroyed automatically.
}

Usage

Must use C++20 or newer.

Install box2d 3.0 as usual, our header includes their.

Clone and add include/ to the header search path. Include <box2c.hpp>. The library is header-only.

Regenerating the header

If the header is outdated, you can regenerate it yourself. (See comment at the beginning of header for underlying box2d version.)

Prerequisites: git, GNU make, sed, perl, gawk (GNU awk). On Windows, install that from MSYS2 and run inside MSYS2 terminal (or use WSL).

Run make. It will clone box2c to ./box2c (or do nothing if already exists; make sure to advance it to the latest commit manually if needed), and will regenerate include/box2c.hpp. The comments at the beginning of Makefile have instructions for running tests, if you want to.

How it works

Ownership

You might have noticed from the example that all .Create...(...) functions accept an extra parameter at the beginning, either b2::OwningHandle or b2::DestroyWithParent.

• w.CreateBody(b2::OwningHandle, ...) returns b2::Body that will destroy the body in its destructor.

  You must save the return value somewhere, or the body will die immediately.

  b2::Body is movable but not copyable. You can destroy it manually without waiting for the destructor using = {}; or .Destroy().

  It's an error to destroy b2::Body after destroying b2::World it was created in (box2d should raise an assertion in debug mode).

• w.CreateBody(b2::DestroyWithParent, ...) returns b2::BodyRef that doesn't destroy the body automatically. Box2d will destroy it with the world. (And shapes and joints created in this mode will be destroyed with the bodies they're attached to.)

  This behavior is useful for entities that you don't want to touch after creation, such as shapes and joints, and perhaps static level geometry.

  You can safely ignore the return value.

  b2::BodyRef is copyable and cheap to copy, pass it around by value. Use it instead of references to b2::Body. Use b2::BodyConstRef instead of const references. (Using references to b2::Body as function parameters is not recommended, because you will be unable to pass b2::BodyRef you get from b2::DestroyWithParent.)

  You can still destroy the object manually by calling .Destroy() on a b2::BodyRef. Note that = {}; for b2::BodyRef just zeroes the reference without destroying the body. It is an error to call .Destroy() on a b2::BodyRef if b2::Body exists for the same object (can only happen if you got it via b2::OwningHandle and then made a b2::BodyRef from it).

All our classes behave similarly. (E.g. there is b2::Shape and b2::ShapeRef for shapes, etc.)

All classes are default-constructible, and are null by default. They are convertible to bool, use that to check for null. There is also .IsValid() that validates the handle through box2d, but it shouldn't be very useful outside of debug assertions.

b2::Body is convertible to b2::BodyRef and b2::BodyConstRef. b2::BodyRef is convertible to b2::BodyConstRef. (And so on.)

Joint classes are a bit special, as they inherit from a common base (e.g. b2::WeldJoint inherits from b2::Joint). You can store them using either the specific derived classes or using the base class. Specific joint types are implicitly convertible to the generic joint, but the inverse (downcast) requires an explicit constructor call that raises an assertion if the actual joint type is wrong.

Parameters structs

We have b2::Body::Params instead of b2BodyDef (and similarly for all other classes).

Those Params classes are automatically set to their default values in the constructor (unlike the original ...Def structs, for which you must call b2Default...Def() to initialize them).

But since we have a constructor, you can no longer use the designated initializer syntax (you can't do b2::Body::Params{.type = ..., .position = ...}, and must do b2::Body::Params p; p.type = ...; p.position = ...;).

As a workaround, if you like oneliners more than you don't like macros, you can do this:

#define ADJUST(x, ...) [&]{auto _ = x; __VA_ARGS__; return _;}

auto params = ADJUST(b2::Body::Params{}, _.type = ..., _.position = ...);

All our functions accept both Params and the original ...Def structs if you must use them.

Using callbacks

Functions accepting callbacks are enhanced to accept lambdas (including capturing lambdas), rather than only function pointers.

The void* context parameter is removed, it's no longer needed when you can use capturing lambdas directly.

When the callback receives a b2ShapeId, you can (and should) instead use b2::ShapeRef or b2::ShapeConstRef instead. (This is const-correct: if called on a const b2::World, b2::ShapeConstRef works and b2::ShapeRef doesn't).

std::string str = "Foo!";
world.Overlap(
    b2Circle{.center{}, .radius = 1},
    b2Transform_identity,
    b2DefaultQueryFilter(),
    [&](b2::ShapeRef shape)
    {
        (void)shape;
        std::cout << str << '\n'; // Lambdas can capture variables.
        return true;
    }
);

Compare with the original C API:

std::string str = "Foo!";
b2Circle circle{.center{}, .radius = 1};
b2World_OverlapCircle(
    world,
    &circle,
    b2Transform_identity,
    b2DefaultQueryFilter(),
    [](b2ShapeId shape, void *context)
    {
        (void)shape;
        // Must pass information through a `void *`.
        std::cout << static_cast<std::string *>(context) << '\n';
        return true;
    },
    &str
);

Those callback improvements are not yet implemented for b2::World::SetPreSolveCallback(), and for functions in b2::DynamicTree.

Debug ImGui renderer

We have an optional debug renderer using ImGui.

See comments in <box2cpp/debug_imgui_renderer.hpp> for usage details.

Simple example: (this assumes you already know how to use ImGui)

#include <box2cpp/box2c.hpp>
#include <box2cpp/debug_imgui_renderer.hpp>

int main()
{
    // Initialize ImGui...

    b2::World w(b2::World::Params{});
    b2::DebugImguiRenderer debug_renderer;

    while (true)
    {
        // ImGui main loop...

        debug_renderer.DrawShapes(world); // Draw Box2D shapes.
        debug_renderer.MouseDrag(world); // Allow dragging bodies with mouse.
        debug_renderer.DrawModeToggles(); // Checkboxes for adjusting visualization.
    }
}