基类具有不完整的类型错误

基类具有不完整的类型

此错误到底是什么意思，我该如何解决？我尝试通过在 EntityPhysics 标头中执行class Entity来向前声明类，但它不起作用。

这是我的实体。

#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__) */

和我的实体物理.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__) */

我认为这可能与我在儿童班中包括Entity.h有关，但如果我这样做，我会得到同样的错误。