CODEWITHSUNDEEP
c++visual-c++abstract-classvirtual-functions
user1673234
user1673234

Reputation: 21

C++ - Unable to instantiate abstract class

(I'm pretty new to C++, so hopefully this is just a rookie mistake)

I'm having problems in my code, where I have a class "Player" that needs a number of attributes, which I'm try to give it though the use of abstract classes as such:

//player.h

class Player : public IUpdate, public IPositionable, public IMoveable, public IDrawable
{
public:
    Player(void);
    SDL_Rect get_position();
    void move(Uint32 dTime);
    void update(Uint32 dTime);
    void show(SDL_Surface* destination);
    ~Player(void);
private:
    SDL_Surface texture;
    int x, y;
};

And I'm overriding the pure virtual functions as such:

//Player.cpp
Player::Player(void)
{
}

SDL_Rect Player::get_position()
{
    SDL_Rect rect;
    rect.h = 0;
    return rect;
}

void Player::move(Uint32 dTime)
{

}

void Player::update(Uint32 dTime)
{
    move(dTime);
}

void Player::show(SDL_Surface* destination)
{
    apply_surface(x, y, &texture, destination, NULL);
}

Player::~Player(void)
{
}

However I keep getting the compilation error: C2259: 'Player' : cannot instantiate abstract class

As far as I can see, the pure virtual functions should be overridden, which my Google-searches have told me, would have made Player non-abstract, yet Player still appears to be abstract.

EDIT: The pure virtual functions:

class IPositionable
{
public:
    virtual SDL_Rect get_position() = 0;
private:
    int posX, posY;
};

class IUpdate
{
public:
    virtual void update (Uint32 dTime) = 0;
};

class IMoveable
{
public:
    int velX, velY;
    virtual void move(Uint32 dTime) = 0;
};

class IDrawable
{
public:
    virtual void show() = 0;
private:
    SDL_Surface texture;
};

class IHitbox
{
    virtual void check_collsion() = 0;
};

class IAnimated
{
    virtual void next_frame() = 0;
    int state, frame;
    int rows, columns;
};

Upvotes: 2

Views: 7932

Answers (5)

Alok Save
Alok Save

Reputation: 206518

Your problem is here: 

class IDrawable
{
public:
    virtual void show() = 0;
};

void Player::show(SDL_Surface* destination)
{
    apply_surface(x, y, &texture, destination, NULL);
}

Note that Player::show(SDL_Surface* destination) does not override the pure virtual method IDrawable::show().
In order to override the method you need to exact same function signature in derived class(only co-variant return types are allowed)
What you have right now is a method named show() in derived class which hides the method named show() in Base class it does not override it. Since you don;t provide definitions for all pure virtual functions of your class Player compiler rightly tells you it is an Abstract class.

Upvotes: 4

Baiyan Huang
Baiyan Huang

Reputation: 6771

Surely it is caused by missing an override of a pure virtual function - maybe just a subtle sigature difference.

I would expect the compiler would tell you which function is still not overriden, something like (vc9):

C2259: 'Player' : cannot instantiate abstract class
due to following members:
'void IUpdate::update(void)' : is abstract
virtualclass.cpp(3) : see declaration of 'IUpdate::update'

If your compiler didn't report this, you could check by removing the inherited interface one by one.

Upvotes: 0

Luc Danton
Luc Danton

Reputation: 35449

It's possible that instead of overriding a pure virtual function of one of the bases, you've instead declared and defined a function with a subtly different signature, as in the following:

struct base {
    virtual void foo(double d) = 0;
};

struct derived: base {
    // does not override base::foo; possible subtle error
    void foo(int i);
}

You might want to double-check your code by reviewing it. If you were using C++11, you'd be able to mark your functions override to catch such errors.

Upvotes: 1

Chen Harel
Chen Harel

Reputation: 10052

In C++ a function is not virtual unless it is specifically written:

virtual void move(Uint32 dTime);

A pure virtual function is defined like this:

virtual void move(Uint32 dTime) = 0;

The "interfaces" you inherit from (notice that this is multiple inheritance.. C++ doesn't differ interfaces from classes) have pure virtual functions you didn't implement thus making your class abstract.

Upvotes: 0

Ed Heal
Ed Heal

Reputation: 59997

An abstract class is abstract - i.e. something is not defined but just declared.

You need to define all those methods. As I do not have the declarations for those classes I cannot advise you on what methods you are missing.

Upvotes: 0

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