Chapter 33. Multidimensional Arrays

As we saw in section 14.7, C and C++ do not support multidimensional arrays of dynamic size, except where all but the most significant dimension is of constant size. In other words, you can do the following:

void f(int x)


{


  new byte_t[x];


}

but not:

void f(int x, int y, int z)


{


  new byte_t[x][y][z]; // Error


}

Creating a multidimensional array can only be in this form:

const size_t Y = 10;


const size_t Z = Y * 2;





void f(int x)


{


  new byte_t[x][Y][Z];


}

This can be quite a restriction. For example, a couple of years ago I was working on a multimedia product user interface where the layout of the interface was dynamically retrieved from a central server over the Internet. The layout could be any possible combination of video, text, dialog, image, and other visual controls within rectangles within rectangles to arbitrary depth sufficient to emulate an unstructured layout. All these levels of rectangles cried out "rectangular array," but because neither dimension was fixed or anticipatable, the built-in array support was not up to the job.

Imperfection: C/C++ does not support dynamically dimensioned multidimensional arrays.

You may not regard this as an imperfection, since one of the principles of C++ is to favor the provision of functionality by the addition of libraries rather than library features, and there exist such libraries, as we'll see later in this chapter. However, as I will demonstrate, it is not possible to fully emulate the syntax of the built-in arrays, although we can come pretty close.

Clearly the answer to this problem lies in custom containers. As we learned in section 14.7, the language supports the taking of slices, so this is eminently feasible. We'll look at several mechanisms for providing N-dimensional arrays in this chapter.^[1]

^[1] One or two chaps have suggested that such a study should include std::valarray in the comparison. Call me a pampered popinjay, but I simply can't bring myself to use a class which overrides operator ==() and returns, not a Boolean-indicating equality, but another valarray whose elements represent the equality of the individual elements of its two comperands!