Accessing property of class instance

Question

Im trying to get a box2d simulation printing out x & y floats to the screen in a similar fashion to the helloWorld example included with the library. I've managed to both build and link to the library.

I've got a class defining a ball that should drop from a point on the screen then fall. But when I try to get the velocity I can't access the members data.

objects.h contents

class Ball {
public:
    bool m_contacting;
    b2Body* m_body;
    float m_radius;

public:
    // Ball class constructor
    Ball(b2World* m_world, float radius) {
    m_contacting = false;
    m_body = NULL;
    m_radius = radius;

    //set up dynamic body, store in class variable
    b2BodyDef myBodyDef;
    myBodyDef.type = b2_dynamicBody;
    myBodyDef.position.Set(0, 20);
    m_body = m_world->CreateBody(&myBodyDef);

    //add circle fixture
    b2CircleShape circleShape;
    circleShape.m_p.Set(0, 0);
    circleShape.m_radius = m_radius; //use class variable
    b2FixtureDef myFixtureDef;
    myFixtureDef.shape = &circleShape;
    myFixtureDef.density = 1;
    myFixtureDef.restitution = 0.83f;
    m_body->CreateFixture(&myFixtureDef);
    m_body->SetUserData( this );
    m_body->SetGravityScale(5);//cancel gravity (use -1 to reverse gravity, etc)
    }
~Ball(){}
};

Instantiation - Ball should now be in simulation

Ball* ball = new Ball(&world, 1);
balls.push_back(ball);

Attempt to print the position and angle of the body.

b2Vec2 position = m_body->GetPosition();
float32 angle = m_body->GetAngle();

printf("%4.2f %4.2f %4.2f\n", position.x, position.y, angle);

The error message declares that m_body is not declared in the scope. Which seems plain enough, if I define a body in the world like b2Body* body; and test against that the code compiles and runs, but then segfaults because I've passed an empty reference. So how can I get access to the properties of my class instance and print them out.

I've tried using b2Vec2 position = Ball::m_body->GetPosition(); & b2Vec2 position = balls->GetPosition(); but no joy.

const_ref const_ref · Accepted Answer · 2014-03-06T12:41:48

m_body is a member of the Ball class and you are trying to access it without using a Ball object. You will need to do something like the following to gain access

ball->m_body->GetPosition();

or access a Ball stored in the vector(assuming you are using c++11)

for(auto& b : balls)
{
    (*b).m_body->GetPosition();
}

or

for(int i = 0; i < balls.size(); ++i)
{
    Ball* b = balls[i];
    b->m_body()->GetPosition();
}

Ideally you should not be using raw pointers and should instead do

Ball ball(&world, 1)
ball.m-body->GetPosition();

or at the very least look into smart pointers(unique_ptr) etc.

Accessing property of class instance

1 Answers