Why is virtual function not being called?

Question

//GUITEXT
class guitext : public entity {
public:
    guitext(graphics *gfx, std::string _text, float _x, float _y, 
        float _size, float timeToLive);
    bool update(float deltaTime, gameworld *world);
    void draw(graphics *gfx);
};

void guitext::draw(graphics *gfx) { printf("draw"); }

//ENTITY

class entity {
public:
    virtual bool update(float deltaTime, gameworld *world) 
        { return false; }
    virtual void draw(graphics *gfx) { }
};

//GAMEWORLD

void gameworld::addEntity(entity e) { entitys.push_back(e); }

//MAIN 

for(int i = 0; i < (int)entitys.size(); i++) { entitys[i].draw(gfx); }

I have a vector in my gameworld class. When I add push a guitext entity to this vector I expect it to call the guitext::draw() function. But the base class function is being called. What am I doing wrong?

possible duplicate of [C++ Overriding Methods](http://stackoverflow.com/questions/1230006/c-overriding-methods) — CB Bailey, Jan 03 '11 at 16:52
Comment on code: Rather than use the correct type you think casting away the problem `(int)entitys.size()` is a good idea! Learn **never** to use the C-Style cast operator and use the C++ case operators only when absolutely necessary. — Martin York, Jan 03 '11 at 18:06

score 6 · Accepted Answer · answered Jan 03 '11 at 14:44

6

You made a vector of entity. Those objects always have type entity. If you want to invoke polymorphism, they need to be pointers or references. How can a vector of entity store a guitext? There's not enough space, it doesn't know how to destroy it, etc etc.

answered Jan 03 '11 at 14:44

Puppy

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@Martin: Someone else already posted an answer with a link to slicing. I'm all for writing wonder answers, but I'm not gonna dupe. – Puppy Jan 03 '11 at 18:23

score 4 · Answer 2 · answered Jan 03 '11 at 14:47

4

Was the vector declared as vector<entity>? Then only the base class part can be stored there, i.e. you lose polymorphism (which only works through pointer or reference in C++).

answered Jan 03 '11 at 14:47

Nikolai Fetissov

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I believe that it only works through pointer or reference in any language, but that most languages have virtually exclusive GC references, like Java or C#. – Puppy Jan 03 '11 at 16:29
Yes, under the hood, but C++ requires explicit syntax. – Nikolai Fetissov Jan 03 '11 at 16:37

score 4 · Answer 3 · edited May 23 '17 at 12:04

4

What you've done is a bit concealed variant of slicing.

edited May 23 '17 at 12:04

Community

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answered Jan 03 '11 at 14:47

Yippie-Ki-Yay

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score 0 · Answer 4 · answered Jan 03 '11 at 15:06

0

You should define entitys to contain pointers to entity. Slightly edited example derived from your code.

#include "stdafx.h"
#include <vector>
#include <string>

class entity 
{
public:
    virtual void draw() { }
};

class guitext : public entity 
{
public:
    void draw()
    {
        printf("draw");
    }
};

int _tmain(int argc, _TCHAR* argv[])
{
    std::vector<entity *> entitys;

    guitext g;

    entitys.push_back(&g); 

    for(int i = 0; i < (int)entitys.size(); i++) 
    { 
        entitys[i]->draw(); 
    }

    return 0;
}

answered Jan 03 '11 at 15:06

Indy9000

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C++'s default entrypoint is int main, and there is no standard _TCHAR in C++. `stdafx.h` is some Microsoft-specific extension. – Lightness Races in Orbit Jan 03 '11 at 16:25
Yes, This is a compilable and runnable example (on Visual Studio) to illustrate the point. – Indy9000 Jan 04 '11 at 13:44

score 0 · Answer 5 · answered Jan 03 '11 at 16:23

0

You're storing Entitys, not pointers to objects of varying derived types of Entity.

answered Jan 03 '11 at 16:23

Lightness Races in Orbit

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score 0 · Answer 6 · answered Jan 03 '11 at 16:36

0

In addition, in your case it's not good idea to pass arguments by value, i suppose there will be very big quantity of objects that need to be redrawed. Better, by const reference, since functon doesn't change state of passed object inside.

answered Jan 03 '11 at 16:36

StNickolay

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Why is virtual function not being called?

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