C++ Dynamic Array Write Access Violation

Question

This has been bugging me for almost 2 days now. I have in my class definition a 2-D dynamic array:

class Raster {

public:
    int pixels[][4];

    void drawTriangle(Vector2f & V1, Vector2f & V2, Vector2f & V3, PixelColor & colorA, PixelColor & colorB, PixelColor & colorC);

};

In my drawing method I have this loop

for (int Y = maxY; Y >= minY; Y--) {
    for (int X = minX; X <= maxX; X++) {
        float lambda1;
        float lambda2;
        float lambda3;
        triangle.getBarycentricCoordinate(X, Y, &lambda1, &lambda2, &lambda3);
        if ((0.0f <= lambda1 && 0.0f <= lambda2 && 0.0f <= lambda3)) {
            PixelColor a = lambda1 * colorA;
            PixelColor b = lambda2 * colorB;
            PixelColor c = lambda3 * colorC;
            PixelColor interpolatedColor = a + b + c;
            pixels[Y*width + X][0] = interpolatedColor.R;
            pixels[Y*width + X][1] = interpolatedColor.G;
            pixels[Y*width + X][2] = interpolatedColor.B;

        }
    }
}

Can anyone point out why it is wrong? Here is the error message: "Exception thrown: write access violation. this was 0x111013530C28FA2."

Please provide a [MCVE](https://stackoverflow.com/help/mcve). — xskxzr, Feb 02 '18 at 04:27
@melpomene Raster::Raster(int w, int h, PixelColor c) { width = w; height = h; clearColor = c; } here's my raster constructor — Manh Nguyen Huu, Feb 02 '18 at 04:27
@melpomene sorry I didnt catch your meaning. What do you mean then? I called the copy constructor by this Raster raster(50, 50, BLACK); — Manh Nguyen Huu, Feb 02 '18 at 04:29
That's not a copy constructor, but OK. That declaration doesn't allocate any storage for `pixels`. What you have is effectively an array of size 0. — melpomene, Feb 02 '18 at 04:31
@immibis OK so it seems like I didnt allocate any memory for pixels. So the memory should be allocated in the constructor right? — Manh Nguyen Huu, Feb 02 '18 at 04:35
You're using a C feature called "flexible array members". As far as I know, this isn't allowed in C++. — melpomene, Feb 02 '18 at 04:36
@melpomene GCC allows it as an extension. (and I imagine so do most other C/C++ compilers that implement it for C) — user253751, Feb 02 '18 at 04:37
@immibis I don't see how it's useful here, though. You'd have to somehow allocate more storage than `sizeof (Raster)` requires (but properly aligned), then manually call the constructor on it. Can you even do that without `malloc`, casting, etc? — melpomene, Feb 02 '18 at 04:38
Related: https://stackoverflow.com/questions/15494588/cpp-empty-array-declaration — melpomene, Feb 02 '18 at 04:41
Depends on what you're trying to do. You haven't posted a [mcve], so it's hard to recommend anything. — melpomene, Feb 02 '18 at 04:42
@melpomene sorry. I'm just trying to create a 2d array with the first dimension of size width*height and 2nd dimension of size 4 — Manh Nguyen Huu, Feb 02 '18 at 04:48
@ManhNguyenHuu -- `int pixels[][4];` -- I wish gcc turned **off** this option by default. It isn't C++, and all it does is lead persons down the wrong path. If you want a dynamic array, use `std::vector`. — PaulMcKenzie, Feb 02 '18 at 05:01

AndreyS Scherbakov · Answer 1 · 2018-02-02T04:30:02.567

0

pixels[][2] doesn't define a non-zero length array here. You need to specify a number for the first dimension too.

edited Feb 02 '18 at 04:30

answered Feb 02 '18 at 04:20

AndreyS Scherbakov

2,674
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Vishaal Shankar · Answer 2 · 2018-02-02T06:02:34.717

I don't think that's a dynamic array.

When you declare an array you are required to declare it's size as well. You can tell what kind of array it is by what you create it as and what the data is. For instance in the following code :

// Here we can see that the array is a 4X3.
int pixels[][3] = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9}, {10, 11, 12} };

This would work just fine, because the size of the array is understood.

Further i would like to add that if you really want something not restrained by size and want the size to be dynamic depending on the data you have, then you could use the various containers that Standard Template Library offers such as a std::vector.

C++ Dynamic Array Write Access Violation

2 Answers2