Function of parent being called

[Werner2]

Hi, why is my program calling the function of the parent instead of the child?

Where the function is being called

std::vector<GameEntity> entity_list(0);

    std::cout << "Entity list created" << std::endl;

    b2Vec2 vs[5]; vs[0].Set(0,0); vs[1].Set(float(DISPLAY_WIDTH/32),0); vs[2].Set(float(DISPLAY_WIDTH/32),float(-DISPLAY_HEIGHT/32));
    vs[3].Set(0,float(-DISPLAY_HEIGHT/32)); vs[0].Set(0,0);

    std::cout << "Wall configuration created." << std::endl;

    StaticWall wall(vs,6, physics_world, "Staticwall"); // StaticWall is supposed to be a child of GameEntity

    std::cout << "Wall initialised." << std::endl;

    entity_list.push_back(wall);
    std::cout << entity_list.size() << std::endl;
    std::cout << "Spawned a wall" << std::endl;
    entity_list[0].draw_self();// <-- This is where the wrong function gets called. 

Declaration of GameEntity and StaticWall

#include "pinball.h"

class GameEntity{
    public:
    std::string test;
    void virtual draw_self(){std::cout << "WRONG! GameEntity draw() called!" << test << std::endl;};
};

class StaticWall : public GameEntity{
    public:

    std::vector<Drawable_Line> drawList;

    StaticWall(b2Vec2 vertices[], int vertex_amount, b2World & physics_world, std::string test){
        b2BodyDef bodyDef;
        bodyDef.type = b2_staticBody;
        bodyDef.position.Set(0,0);

        b2Body * body = physics_world.CreateBody(&bodyDef);

        b2ChainShape chain;
        chain.CreateChain(vertices, vertex_amount);

        b2FixtureDef fixtureDef;
        fixtureDef.shape = &chain;
        fixtureDef.density = 1.0f;
        fixtureDef.friction = 0.3f;
        body->CreateFixture(&fixtureDef);
        std::cout << "(Wall Init) Teststring: " << test << std::endl;
        this->test = test;
        std::cout << "(Wall Init) Filling draw list." << std::endl;

        drawList = std::vector<Drawable_Line>(5);

        for (int i=0; i<vertex_amount-1;i++){
            Drawable_Line line(vertices[i].x,vertices[i].y,vertices[i+1].x,vertices[i+1].y);
            std::cout << "(Wall Init) Created draw line!." << std::endl;
            drawList[i] = line;
            std::cout << "(Wall Init) Draw line added to draw list!" << std::endl;
        }

        std::cout << "(Wall Init) Finished!." << std::endl;
    }

    void draw_self(){
        std::cout << "Drawing static wall" << test << std::endl;

        for (unsigned i=0; i < drawList.size(); i++) {
            drawList[i].draw();
        }
    }

};

[cire]

std::vector<GameEntity> entity_list(0);

This means that everything in entity_list is a GameEntity. Because anything in the vector cannot be anything but a GameEntity, methods called on objects in the vector will always be those of GameEntity.

To get the behavior you want, entity_list should contain pointers or references to GameEntity.

StaticWall wall(vs,6, physics_world, "Staticwall"); // StaticWall is supposed to be a child of GameEntity

  entity_list.push_back(wall);

This is sometimes called shearing. The derived portion of the instance is simply lost when it's fed to a function taking an argument of type BaseClass.

[Werner2]

Ok, that's one difference from Java...

But this doesn't work: std::vector<*GameEntity> entity_list(0);

I get a compile error.

If I try to put *s in other places, it gives me other errors. How do I change my code to store pointers in the vector like you said?

[cire]

std::vector<GameEntity*> entity_list ;

However, you must make sure the objects which are pointed to aren't deallocated before the pointer is removed from entity_list or entity_list is destroyed.

In the first block of code it looks like the lifetime of entity_list and the wall you created would be the same, but it is something you should be aware of.

[Werner2]

Ah, * after GameEntity, not before...

Ok, probably a complete noob question here: Does the data that the pointer points to have to be allocated on the heap?=

[cire]

No, it does not.