2D Array of Pointers -> Classes

Question

I am having problems with my constructor in class World.

I created a 2D array with pointers where each entry in the array is of type Organism, hence the line of code:

Organism* grid[20][20];

When I run my program, I only see

hello

and after that, I get a message saying that my program has stopped working. I'm pretty sure it's the line of code

grid[i][j]->symbol = ' ';

that's causing the problem. Just to see what would happen, I changed that line to

grid[i][j];

and didn't get any errors. But, the moment I put ->, I seem to get errors.

Is there a reason why my program stops working after I put ->? Any help would be appreciated.

This is my code:

#include <iostream>

using namespace std;

class Organism
{
public: 
    char symbol;
};

class World
{
public:
    World();

private:
    Organism* grid[20][20];
};


int main()
{
    World world;
    return 0;
}


World::World()
{
    for(int i = 0; i < 20; i++)
        for(int j = 0; j < 20; j++)
        {
            cout << "hello" << endl;
            grid[i][j]->symbol = ' ';
            cout << "here" << endl;
        }
}

You have an array of pointers to `Organism` that don't point to anything. — user657267, Jul 30 '14 at 08:43
Yeah, how is the `->` operator supposed to dereference a pointer if there's no memory allocated for the object? — The Paramagnetic Croissant, Jul 30 '14 at 08:43
I guess my problem is that I don't fully understand pointers. — enochsoccer, Jul 30 '14 at 08:53
@enochsoccer a pointer tells you whereabouts in memory something else can be found. It does not imply any sort of ownership. — M.M, Jul 30 '14 at 08:55

score 0 · Accepted Answer · edited May 23 '17 at 12:04

You have an array of pointers to Organism.

Pointers can not hold any data other than an address. They can only point to memory (that holds data).

Your array's pointers does not point to anything, thats why you get undefined behaviour when you try to assign data to where the pointers point at.

You need to allocate memory for your array.

Organism grid[20][20]; // Create an array of objects (not pointers).
/* ... */
grid[i][j].symbol = ' ';

Same using dynamic memory:

class World {
public:
    World();
    ~World();                                // Rule of Three.
    World(const World&) = delete;            // Rule of Three.
    World& operator=(const World&) = delete; // Rule of Three.
private:
    Organism** grid;
};

World::World() {
    grid = new Organism*[20]; // Allocate memory to point to.
    for(std::size_t i = 0; i != 20; ++i) {
        grid[i] = new Organism[20]; // Allocate memory to point to.
        for(std::size_t j = 0; j != 20; ++j) {
            cout << "hello" << endl;
            grid[i][j].symbol = ' ';
            cout << "here" << endl;
        }
    }
}

// Destructor needed to deallocate memory (otherwise it will leak).
World::~World()
{
    for (std::size_t i = 0; i != 20; ++i) {
        delete[] grid[i];
    }
    delete[] grid;
}

Now you can see how complicated it gets when using dynamic memory and why it's recommended to prefer to use automatic storage duration (i.e. create objects, not pointers and new/delete).

Even better is to use a container from the standard library for storing your elements as e.g. std::vector.

Related:

What is The Rule of Three?

Okay, that makes more sense, but now, I get the error: invalid abstract type 'Organism' for 'grid' — enochsoccer, Jul 30 '14 at 09:04

John Doe · Answer 2 · 2014-07-30T09:12:22.410

0

Change

Organism* grid[20][20];

To

Organism  grid[20][20];

and use

grid[i][j].symbol = '';

instead of

grid[i][j]->symbol = '';

and add a default constructor to Organism

class Organism
{
   Organism();
   ...
}

or make Organism a struct

struct Oranism
{
 ...
}

edited Jul 30 '14 at 09:12

answered Jul 30 '14 at 08:59

John Doe

146
4

Now, I get the error: invalid abstract type 'Organism' for 'grid' – enochsoccer Jul 30 '14 at 09:04
i guess you should add a default constructor for Organism or change it from class to struct. – John Doe Jul 30 '14 at 09:07

2D Array of Pointers -> Classes

2 Answers2