Keeping User Data for Box2D

main.cpp

// https://coliru.stacked-crooked.com/a/7765d15fe603a3d9
// https://gamedev.stackexchange.com/a/196955/115807

#include <iostream>
#include <string>
#include <vector>
#include <memory>
#include <map>

struct b2Fixture;

struct MyFixtureUserDataType
{
  int myVal {};
  b2Fixture* mFixture {};
};

struct b2FixtureUserData
{
  b2FixtureUserData()
  {
    pointer = 0;
  }

  uintptr_t pointer;
};


struct b2FixtureDef
{
  b2FixtureUserData userData;
};

struct b2Fixture
{
  b2Fixture( const b2FixtureDef* def )
  {
    m_userData = def->userData;
  }

  b2FixtureUserData& GetUserData() { return m_userData; }
  b2FixtureUserData m_userData;
};

int main()
{
  std::map<uintptr_t, std::unique_ptr<MyFixtureUserDataType>> allMyThings;

  auto myThing = std::make_unique<MyFixtureUserDataType>();
  myThing->myVal = 3;

  b2FixtureDef def;
  def.userData.pointer = reinterpret_cast<uintptr_t>( myThing.get() );


  std::unique_ptr<b2Fixture> fix = std::unique_ptr<b2Fixture>( new b2Fixture( &def ) ); // we do this for the sake of this example.
  myThing->mFixture = fix.get();

  allMyThings[reinterpret_cast<uintptr_t>( fix.get() )] = std::move( myThing );
  myThing = nullptr;

  // do something to access your data
  std::cout << reinterpret_cast<MyFixtureUserDataType*>( fix->GetUserData().pointer )->myVal;

  // We no longer need it
  fix.reset();
  allMyThings.erase( reinterpret_cast<uintptr_t>( fix.get() ) );
}