Base class has incomplete type error

Question

Base class has incomplete type

What exactly does this error mean and how can I fix it? I have tried forward declaring the class by doing class Entity in my EntityPhysics header but it did not work.

Here is my Entity.h

#ifndef __Game__Entity__
#define __Game__Entity__

#include <iostream>
#include <string>

#include "OGRE/Ogre.h"

#include "OgreInit.h"

class Entity{
public:
    Entity(std::string entityId, std::string mesh, Ogre::Vector3 position = Ogre::Vector3::ZERO, Ogre::Vector3 rotation = Ogre::Vector3::ZERO);
    virtual ~Entity() = 0;

    void setPosition(Ogre::Vector3 position);
    Ogre::Vector3 getPosition();
    void setRotation(Ogre::Vector3 rotationIncrease);
    Ogre::Vector3 getRotation();
    void setMesh(std::string meshName);
    std::string getMesh();
    virtual void tick() = 0;
    void removeEntity();

    Ogre::Entity getEntity();
    Ogre::SceneNode getSceneNode();

    std::string entityId;
protected:
    Ogre::Entity *ent;
    Ogre::SceneNode *nod;
};

#endif /* defined(__Game__Entity__) */

And my EntityPhysics.h

#ifndef __Game__EntityPhysics__
#define __Game__EntityPhysics__

#include <iostream>
#include <string>
#include "OGRE/Ogre.h"
#include "OgreBulletCollisionsBoxShape.h"
#include "OgreBulletDynamicsRigidBody.h"

#include "Entity.h"
#include "OgreInit.h"

class EntityPhysics: public Entity //error occurs here: "Base class has incomplete type"
{
public:
    EntityPhysics(std::string pentityId, std::string mesh, Ogre::Vector3 position, Ogre::Vector3 rotation, /*Physics Specific "stuff"*/std::string shapeForm = "BoxShape", float friction = 1.0, float restitution = 0.0, float mass = 1.0);
    virtual ~EntityPhysics() = 0;
    virtual void tick() = 0;
private:
    float friction, restitution, mass;

    OgreBulletCollisions::CollisionShape *collisionShape;
    OgreBulletDynamics::RigidBody *rigidBody;
};

#endif /* defined(__Game__EntityPhysics__) */

I think it may have to do with me including Entity.h in the child class, but if I do that I get the same error.

Answer 1

I had a similar error message which i resolved by moving the #includes lower down the header file so that the class definition and method definition that the other #includes desire come earlier than the other header includes.

Answer 2

This is most likely due to a circular include, and the way to fix this is to remove includes where you don't need them.

In Entity.h you don't need:

#include "OGRE/Ogre.h"
#include "OgreInit.h"

You can, and should, instead forward-declare the types. Same for EntityPhysics.h and:

#include "OGRE/Ogre.h"
#include "OgreBulletCollisionsBoxShape.h"
#include "OgreBulletDynamicsRigidBody.h"

#include "OgreInit.h"

the only one you actually need is Entity.h.