title: C++, help extending this class
body: <p>I'm to extend a C++ class, but have absolutely no background in that language. Googling didn't help me understand how to solve the compilation error:</p>
  
  <pre><code>Constructor for 'JRB2World' must explicitly initialize the base class 'b2World' which does not have a default constructor
  </code></pre>
  
  <p>So, there is this b2World.h</p>
  
  <pre><code>class b2World
  {
  public:
      b2World(const b2Vec2&amp; gravity);
      ~b2World();
  
      // ...
  </code></pre>
  
  <p>And its .cpp:</p>
  
  <pre><code>b2World::b2World(const b2Vec2&amp; gravity)
  {
      // ...
  }
  
  b2World::~b2World()
  {
      // ...
  }
  </code></pre>
  
  <p>My class header:</p>
  
  <pre><code>#import "Box2D.h"
  
  class JRB2World : public b2World {
  
      float factor;
  
  public:
  
      JRB2World(const b2Vec2&amp; gravity);
      ~JRB2World();
  
      float getFactor();
      void setFactor(float f);
  };
  </code></pre>
  
  <p>My class implementation:</p>
  
  <pre><code>JRB2World::JRB2World(const b2Vec2&amp; gravity) {
  
  // Constructor for 'JRB2World' must explicitly initialize the base class 'b2World' which does not have a default constructor
  }
  
  JRB2World::~JRB2World() {
  }
  
  float JRB2World::getFactor(){
  
      return factor;
  }
  
  void JRB2World::setFactor(float f){
  
      factor = f;
  }
  </code></pre>
  
  <p>I suppose it has to do with a call to the "super constructor" like in java or objc. How can this be done?</p>