Storing objects in 2 dimensional sprite matrix in c++

Question

I want to store a list of tiles (pointers to objects of class "Sprite") in a two dimensional vector.

Currently I'm storing all the sprites of my tilemap in a vector.

std::vector<Sprite*> _tiles;

Now I need to get all the neighbored tiles and I thought it be easier, to store my sprites in a 2d matrix (vector in vector) to do those caluclations.

But I can't figure out how to do that.

Header:

private:
  std::vector<std::vector<Sprite*>> matrix;

C++ File:

vectorMatrix::vectorMatrix(int columns, int rows) { }

vectorMatrix::~vectorMatrix() { } 


void vectorMatrix::addCellAt(int x, int y, Sprite* sprite) {
  std::vector< std::vector<Sprite*> > matrix;
  matrix[x][y].push_back(sprite);
}

But I get an error message if I use two index operators.

`matrix[x][y]` gives you a `Sprite *`. You should probably assign to it using `=` instead, and switch to using `std::shared_ptr` to avoid memory leaks from not freeing the data. — hnefatl, Apr 28 '18 at 17:10

Some programmer dude · Answer 1 · 2018-04-28T17:18:57.753

1

Three problems:

You declare a local variable matrix inside the function, which shadows the member variable.
If the size of the vectors is not set to include the x and y indexes then you will go out of bounds.
matrix[x][y] is not itself a vector, it's an element that you can assign directly:
```
matrix[x][y] = sprite;
```
Remember to consider problem 2 before doing this.

edited Apr 28 '18 at 17:18

answered Apr 28 '18 at 17:11

Some programmer dude

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the local matrix was a copy and paste error... sry - thanks for clarifying! – Lisa Apr 28 '18 at 17:36

score 1 · Answer 2 · answered Apr 28 '18 at 17:18

1

Why do you think it's a good idea to make your matrix a vector-of-vectors? True, this will "work" in the sense that vec_of_vec_matrix[i][j] will return the right thing, but it's somewhat cumbersome and inefficient.

Instead, consider basing your class on a single vector - of all data, as done in this answer (and probably in many libraries). Element access will be something like (for column-major data):

Sprite*& Matrix::operator()(size_t i, size_t j)
{
    return mData[i * num_columns + j];
}

You could even arrange with map[x][y] to work, using a row proxy class.

answered Apr 28 '18 at 17:18

einpoklum

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Thank you! I found this approach in a lot of examples, that's why i wanted to use it. i don't think it's too cumbersome, but if there's a more performant approach, i'll have a look. Is the performance difference significant? think i'll call a few operations on that tilematrix each frame, so that'd be nice to consider. – Lisa Apr 28 '18 at 18:16
@Chris: It will very likely be perform better because element access will not require double-indirection, i.e. you'll only have one memory load, while the multiply+add is well catered for by CPUs. – einpoklum Apr 28 '18 at 18:19

score 1 · Accepted Answer · answered Apr 28 '18 at 17:27

std::vector< std::vector<Sprite*> > matrix;

This is an empty std::vector of of std::vector, and as others have pointed out, it's no longer the data member matrix.

Since it's empty, it's not possible to do matrix[0] - get the first row, matrix[1] - get the second row... You can however allocate the size of the matrix in your constructor first -- e.g. This will give you a rows*columns matrix

vectorMatrix::vectorMatrix(int columns, int rows) 
  : matrix(rows, std::vector<Sprite*>(columns))
{
}

And then you can set the xth and yth element to the corresponding Sprite*, and of course x and y should be smaller than rows and columns

matrix[x][y] = sprite;

And don't declare matrix again inside the addCellAt, You can use the data members directly anywhere inside the class.

Storing objects in 2 dimensional sprite matrix in c++

3 Answers3