生命游戏，C ++-多态数组（可能还有指针）

Question

I'm having trouble with some code, dealing with polymorphism.

Essentially I am to make a Game of Life, and as such have a driver where I create a grid of Organism pointers (the super class to actual animal classes, in this case, Ant).

In my driver, I create a new Ant in the array of Organism pointers using the following code:

[DRIVER]
Ant *ant = new Ant(&world, GRID_HEIGHT, GRID_WIDTH);
world.setOrganism(ant,4,4); 

Here I create a pointer to an Ant, and plop it in the world. Here's setOrgamism function:

[World]
void World::setOrganism(Organism *organism, int x, int y)
{   grid[x][y] = organism; }

So, this code works when called from the driver. I can make a new ant using this anywhere, no problems.

However, when I try to run this in the Ant's spawn function, I get an Access Violation error and my program crashes executing setOrganism.

Here's the code in my Ant class for the spawn function:

[Ant]
void Ant::spawn(int spawnX, int spawnY)
{
     Ant *newAnt = new Ant(world, GRID_HEIGHT, GRID_WIDTH);
     newAnt->setPosition(spawnX,spawnY);

     world->setOrganism(newAnt, spawnX, spawnY);
}

Any and all help is certainly appreciated!

Edit:

[World.cpp]
#include "World.h"
#include "Ant.h"
#include "Lion.h"

using namespace std;

World::~World(void)
{
}

World::World(void)
{
    for(int y = 0; y < GRID_HEIGHT; y++)
    {
        for(int x = 0; x < GRID_WIDTH; x++)
        {
            grid[x][y] = NULL;
        }
    }
}

Organism* World::getOrganism(int x, int y)
{
    return grid[x][y];
}

void World::setOrganism(Organism *organism, int x, int y)
{
    grid[x][y] = organism;
    organism->setPosition(x,y);
}

_

[Ant]
#include "Ant.h"

using namespace std;

Ant::~Ant(void)
{
}

Ant::Ant(void)
{
}

Ant::Ant(World *world, int width, int height)
{
    srand(time(NULL));
    this->width = width;
    this->height = height;
}

void Ant::spawn(int spawnX, int spawnY)
{
    Ant *newAnt = new Ant(world, GRID_HEIGHT, GRID_WIDTH);
    newAnt->setPosition(spawnX,spawnY);

    world->setOrganism(newAnt, spawnX, spawnY);
}

_

[Driver]
#include "Organism.h"
#include "Ant.h"
#include "Lion.h"
#include "World.h"

using namespace std;

int main()
{

    World *world = new World();

    //This works
    Ant *ant = new Ant(world, GRID_HEIGHT, GRID_WIDTH);
    world->setOrganism(ant,4,4);

    //This does not
    ant->spawn(6,6);
}

If you need any more information, please ask!

Answer 1

You haven't shown the definition of Organism , but from your comments, I assume it has a member World *world . You're passing a World *world into the constructor of Ant , but you're not using it for anything; you're not assigning it anywhere, and you're not calling any non-default constructor of Organism either. This means that the member world is most probably left uninitialised.

This in turn means that inside Ant::spawn , the world (referring to this->world ) is either a null pointer or a garbage value (depending on how the default constructor of Organism initialises its world member). Either of these will lead to an Access violation when dereferenced - precisely what you're getting.

To fix that, make sure Ant 's constructor initialises the world member somehow - probably by passing on the world parameter to the appropriate constructor of Organism .

Answer 2

you didn't declare the type of grid. you just have grid[x][y] = null; you should have set it to Organism grid[x][y], and then set it later on