c++ vector with object pointers

Question

i have to implement a small and simple game in c++ (a maze) and I have some problems right now.

Some snippets: I've got an array of object pointers which represents my fields in the maze

Field*** maze;

init of the maze:

for (n = 0; n < MAZE_WIDTH; n++) {
    this->maze[n] = new Field*[MAZE_HEIGHT];
    for (p = 0; p < MAZE_HEIGHT; p++) {
        this->maze[n][p] = new Field();
        this->maze[n][p]->x = n;
        this->maze[n][p]->y = p;
    }
}

When creating the maze i need a list of already visited fields and a stack so I did:

std::vector<Field*> visited;
std::vector<Field*> stack;

Then later I want to put a Field* into my stack

stack.push_back(neighbour);

But after this push all values in the object are wrong. Even if i try

neighbour = stack.back();

all the values are completly different

I already red some threads about this topic and that's why i chose a vector of pointers and not objects.

Where is my fault?

Edit: Some more snippets as requested:

Of course I allocate memory for the mate itself

this->maze = new Field**[MAZE_WIDTH];

Field is a simple class which looks like:

class Field {
public:
    Field();
~Field();
bool w_left;
bool w_right;
bool w_front;
bool w_back;
unsigned int x;
unsigned int y;
private:
};

You are not by chance modifying the same Field reference every time when adding to the stack? This would cause all the values to be the same and equal to the last value added. — Tudor
– Tudor, Commented Jan 4, 2012 at 10:50
Could you give a little more code? What is the type of neighbour, for example? How is it initialized? (It sounds like you might be inserting a pointer to a local variable into the vector, but without more code, it's impossible to tell.) — James Kanze
– James Kanze, Commented Jan 4, 2012 at 10:50
It may be the code snippet, but did you miss the new Field**[] at the beginning ? Also, I think there is simpler ways to handle multidimensional arrays than a pointer to a pointer to a pointer. — slaphappy
– slaphappy, Commented Jan 4, 2012 at 10:51
¤ -1 One glaring error is that you don't allocate anything for the maze pointer. Another big error is having that pointer in the first place. Folks who do that are commonly to as "three srtar programmers. Use e.g. std::vector. Regarding what it seems that your question is about, namely why things don't work, no real answer appears to be possible given the info you present. It is in the direction of "my program does not Work, here are some unrelated bugs, what are the rest of my bugs?". Difficult to answer, to say the least. Cheers & hth., — Cheers and hth. - Alf
– Cheers and hth. - Alf, Commented Jan 4, 2012 at 10:53
You should practically never need a *** in a C program; let alone in C++. — Fred Foo
– Fred Foo, Commented Jan 4, 2012 at 10:55

João Augusto · Accepted Answer · 2012-01-04 17:44:06Z

2

Since, you didn't posted the code of how you are obtaining the values, compare to this, and try to find your problem...

std::vector<std::vector<Field*> > maze;

    // Ini
    for(int i = 0; i < MAZE_WIDTH; i++)
    {
        maze.push_back(std::vector<Field*>());
        for(int j = 0; j < MAZE_HEIGHT; j++) 
        {
            maze[i].push_back(new Field());
            maze[i][j]->x = i;
            maze[i][j]->y = j;
        }
    }

    std::vector<Field*> visited;

// push the field [4,5] in a visited vector

    visited.push_back(maze[4][5]);


    // Clean up
    for(size_t i = 0; i < maze.size(); i++)
    {
        for(size_t j = 0; j < maze[i].size(); j++)      
            delete maze[i][j];      
    }

edited Jan 4, 2012 at 17:44

answered Jan 4, 2012 at 15:29

João Augusto

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1 Comment

soupdiver Over a year ago

yeah i also ended with this solution _)

Luchian Grigore · Accepted Answer · 2012-01-04 10:54:00Z

0

Why declare the maze as Field***?

The C++ alternative is std::vector<std::vector<Field*> > maze;, and that's what you should use.

answered Jan 4, 2012 at 10:54

Luchian Grigore

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4 Comments

codeling Over a year ago

or, even better: std::vector<std::vector<std::shared_ptr<Field> > > maze if C++11 is available, or with boost::shared_ptr/std::tr1::shared_ptr instead if it isn't.

Errata Over a year ago

Even better: MAZE_HEIGHT and MAZE_WIDTH seems to be constant => Field maze[MAZE_WIDTH][MAZE_HEIGHT];. Then when you need to insert in vector stack.push_back(&maze[x][y]);

soupdiver Over a year ago

I declared the maze as you said but I still get the same error when I try to stack.push_back(neighbour)

Mooing Duck Over a year ago

@skelle: was neighbor allocated with new, or on the stack

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