invalid use of incomplete type / forward declaration

Question

I tried to look at the similar problems listed here on Stackoverflow and on Google but they deal mostly with templates and that's not my case. I'm using GCC 4.4.5 on Debian Testing 64bit.
So, I have two classes - CEntity:

#ifndef CENTITY_H_INCLUDED
#define CENTITY_H_INCLUDED

#include "global_includes.h"

// game
#include "CAnimation.h"
#include "Vars.h"
#include "vector2f.h"
#include "Utils.h"

class CAnimation;

class CEntity
{
public:
    CEntity();
    virtual ~CEntity();

    void _update(Uint32 dt);

    void updateAnimation(Uint32 dt);

    void addAnimation(const std::string& name, CAnimation* anim);
    void addAnimation(const std::string& name, const CAnimation& anim);
    void removeAnimation(const std::string& name);
    void clearAnimations();

    bool setAnimation(const std::string& name);

    SDL_Surface* getImage() const;

    const vector2f& getPos() const;
    const vector2f& getLastPos() const;
    F getX() const;
    F getY() const;
    F getLastX() const;
    F getLastY() const;
    SDL_Rect* getHitbox() const;
    SDL_Rect* getRect() const;

    F getXSpeed() const;
    F getYSpeed() const;

    void setPos(const vector2f& pos);
    void setPos(F x, F y);
    void setPos(F n);
    void setX(F x);
    void setY(F y);

    void setHitboxSize(int w, int h);
    void setHitboxSize(SDL_Rect* rect);
    void setHitboxWidth(int w);
    void setHitboxHeight(int h);

    void setSpeed(F xSpeed, F ySpeed);
    void setXSpeed(F xSpeed);
    void setYSpeed(F ySpeed);

    void stop();
    void stopX();
    void stopY();

    void affectByGravity(bool affect);

    void translate(const vector2f& offset);
    void translate(F x, F y);

    bool collide(CEntity& s);
    bool collide(CEntity* s);

protected:
    CAnimation* mCurrentAnimation;
    SDL_Surface* mImage;

    vector2f mPos;
    vector2f mLastPos;
    SDL_Rect* mHitbox; // used for collisions
    SDL_Rect* mRect; // used only for blitting

    F mXSpeed;
    F mYSpeed;

    bool mAffByGrav;

    int mHOffset;
    int mVOffset;

private:
    std::map<std::string, CAnimation*> mAnims;
};

#endif // CENTITY_H_INCLUDED

and CPlayerChar which inherits from CEntity:

#ifndef CPLAYERCHAR_H_INCLUDED
#define CPLAYERCHAR_H_INCLUDED

#include "global_includes.h"

// game
#include "CEntity.h"

class CEntity;

class CPlayerChar : public CEntity
{
public:
    CPlayerChar();
    virtual ~CPlayerChar();

    virtual void update(Uint32 dt) = 0;

    virtual void runLeft() = 0;
    virtual void runRight() = 0;
    virtual void stopRunLeft() = 0;
    virtual void stopRunRight() = 0;

    virtual void attack() = 0;
    virtual void stopAttack() = 0;

    virtual void attack2() = 0;
    virtual void stopAttack2() = 0;

    virtual void ground() = 0;
    virtual void midair() = 0;

    void jump();
    void stopJump();

protected:
    // looking right?
    bool mRight;

    bool mJumping;
    bool mOnGround;
    bool mGrounded;
};

#endif // CPLAYERCHAR_H_INCLUDED

When I try to compile it, GCC throws this error:

CPlayerChar.h:12: error: invalid use of incomplete type ‘struct CEntity’
CPlayerChar.h:9: error: forward declaration of ‘struct CEntity’

I tried it first without the forward declaration 'class CEntity;' in CPlayerChar.h on line 9, but then it would throw this instead

CPlayerChar.h:12: error: expected class-name before ‘{’ token

So the forward declaration has to be there. Also, CEntity is clearly a class, not a struct.

In C++ classes and structs are almost identical; don't get hung up on the error message. — Mark Ransom
I presume that the class definition for CEntity is in the file CEntity.h? If not, that's your problem. — Mark Ransom
You have some circular inclusion in your header files. A header file should only include other header files if it explicitly needs too. If you can use a forward declaration then this should be preferred. — Martin York

Martin York Martin York · Accepted Answer · 2011-08-08T22:31:37

You have a circular inclusion in your header files.
But without all the header files we will not be able to fix it.

I would start here.

#include "CAnimation.h"

Looking at your header you don't actually need this. You only use CAnimation by reference or pointer so the forward declaration you have should be sufficient. Move the include into the source file (ie out of the header).

The next place I would look is:

#include "global_includes.h"

Any global includes that are included in a header file better be very simple. Should only contain simple types and not include any other header files (unless they are just as simple). Anything complex is going to lead to problems with circular dependencies.

General rule of thumb

A header file should only include header files that it absolutely needs. Otherwise they should be included from the source file. You only absolutely need a header file if it defines a class that is used as a parent class you have members objects of that class, or you use parameter objects of that class.

I use the term object to distinguish from references or pointers. If you are using these you do not need to include the header file. You only need to do a forward declaration.

invalid use of incomplete type / forward declaration

3 Answers

General rule of thumb