SwiftBox2D

This is an example of how to use an external C++ Library (from homebrew) with Swift using C++-Interop.

The library used here is box2d (a 2D physics engine for games).

Steps 1, install box2d:

brew install box2d

Steps 2, clone this repo:

git clone https://github.com/plotfi/SwiftBox2D

Step 3, set your homebrew path environment variable:

I put my homebrew in ~/opt/homebrew because I really dislike how homebrew modifies the sytsem library layout. I also really dislike that x64 homebrew is in /usr/local and arm64 homebrew is in /opt some place. Also, my employer's packaging system uses opt as well which makes homebrew in default dirs even more problematic.

Anyhow, I set mine to export BREWDIR=$HOME/opt/homebrew, set yours however you'd like.

Steps 4, annotate types as reference types as needed, also copy module.modulemap files:

At this point, for this particular library one of the class/struct types has to be a reference type. Ideally you'd want to wire up ARC but here we are going to use immortal foreign references for brevity.

All you'll need to do is modify the line class B2_API b2Body at $BREWDIR/Cellar/box2d/2.4.1/include/box2d/b2_body.h to class B2_API IMMORTAL_REF b2Body so that it looks like so:

/// A rigid body. These are created via b2World::CreateBody.
class B2_API IMMORTAL_REF b2Body
{
public:
	/// Creates a fixture and attach it to this body. Use this function if you need
	/// to set some fixture parameters, like friction. Otherwise you can create the
	/// fixture directly from a shape.

You also need to modularize C++ libraries you wish to use from Swift. There is an existing modulemap file at box2d-modifications/module.modulemap in the cloned repository with the contents:

module box2d {
  header "box2d/box2d.h"
  requires cplusplus
  export *
}

copy it to the include dir in your homebrew setup like so:

cp box2d-modifications/module.modulemap $BREWDIR/include

Step 5, build and run:

make
./Box2DHello.exe

The output should look like:

time step index: 42
0.0 1.3722225 0.0
time step index: 43
0.0 1.2500004 0.0
time step index: 44
0.0 1.1250004 0.0
time step index: 45
4.0935977e-14 1.0145822 0.00014126342
time step index: 46
-7.3666545e-11 1.0146508 0.0001100473
time step index: 47
4.118562e-09 1.0147082 8.584162e-05

Notes

You might notice the box2dTypeConvert C++ module. This exists to handle the type conversions between parent and child types in the C++ hierarchy and to work around bugs. It is what it is.