如何将三维数据用作类属性？

Question

It's been a long time since I worked with C++, but I have a class that uses 3-dimensional data and I can't figure out how I can make this work. I need the sizes of the dimensions to be defined in the constructor. I tried this in the header:

class CImage
{
public:
    float values[][][];
...
}

and this in the constructor:

CImage::CImage(int cols, int rows, int depth)
{
    values[cols][rows][depth];
}

but this returns the error: "declaration of `values' as multidimensional array must have bounds for all dimensions except the first".

Also using this in the constructor does not work:

values = new float[cols][rows][depth];

I also tried using vector, but without much success. Header:

vector<vector<vector<float> > > values;

Nothing in constructor. No compiler errors, but when I try to set a value:

values[c][r][d] = value;

the program crashes.

It seems so basic, but I just can't figure it out...

Answer 1

The program crashes when accessing that vector because it is empty, ie there are no elements at those indexes.

The best way to go about this is to make a linear, one-dimensional, vector (or even an array), and access it with a pair of operator()'s, see C++FAQ Lite for details. Or use boost::multi_array .

For example:

#include <vector>
#include <iostream>
class CImage
{
        int X, Y, Z;
        std::vector<float> values;
public:
        CImage(int cols, int rows, int depth)
                : X(cols), Y(rows), Z(depth),
                values(cols*rows*depth) {}
        float operator()(int x, int y, int z) const
        {
                return values[Z*Y*x + Z*y + z];
                // or you lay it out differently in memory
                // if you please, transparent to the user:
                // return values[x + X*y + X*Y*z];
        }
        float& operator()(int x, int y, int z)
        {
                return values[Z*Y*x + Z*y + z];
                // likewise, in a different layout
                // return values[x + X*y + X*Y*z];
        }
};

int main()
{
        CImage ci(3,3,3);
        ci(2,2,2) = 7.0;
        std::cout << ci(2,2,2) << '\n';
}

Answer 2

Just to poiny out why Cubbi is right, this would be the constructor of the 3d-vector:

vector<vector&ltvector&ltfloat>>> values;
// create vector [dim0][dim1][dim2]
// init value: init
size_t dim0 = 3;
size_t dim1 = 3;
size_t dim2 = 3;
float init = 0.42f;
values = vector<vector<vector<float>>>
         (
            dim0,
            vector<vector<float>>
            (
                dim1,
                vector<float>
                (
                    dim0,
                    init
                )
            )
         );

Nice, isn't it?

Besides, you cannot declare a float values[][][]; because arrays reside on the stack, so the compiler has to know at compile time what size that array has (exception to this: C99 variable length arrays, but this would'nt be C++). You could declare a float*** values; (using new float[c][r][d]; in the ctor), but this is awful, too.