c++ - Accessing property of class instance -

im trying box2d simulation printing out x & y floats screen in similar fashion helloworld example included library. i've managed both build , link library.

i've got class defining ball should drop point on screen fall. when try velocity can't access 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 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( );     m_body->setgravityscale(5);//cancel gravity (use -1 reverse gravity, etc)     } ~ball(){} }; 

instantiation - ball should in simulation

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

attempt print position , angle of 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 m_body not declared in scope. seems plain enough, if define body in world b2body* body; , test against code compiles , runs, segfaults because i've passed empty reference. how can access properties of class instance , print them out.

i've tried using b2vec2 position = ball::m_body->getposition(); & b2vec2 position = balls->getposition(); no joy.

m_body member of ball class , trying access without using ball object. need following gain access

ball->m_body->getposition(); 

or access ball stored in vector(assuming using c++11)

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

or

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

ideally should not using raw pointers , should instead do

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

or @ least smart pointers(unique_ptr) etc.