Chapter 19. Boost.MultiArray

Boost.MultiArray is a library that simplifies using arrays with multiple dimensions. The most important advantage is that multidimensional arrays can be used like containers from the standard library. For example, there are member functions, such as begin() and end(), that let you access elements in multidimensional arrays through iterators. Iterators are easier to use than the pointers normally used with C arrays, especially with arrays that have many dimensions.

#include <boost/multi_array.hpp>
#include <iostream>

int main()
{
  boost::multi_array<char, 1> a{boost::extents[6]};

  a[0] = 'B';
  a[1] = 'o';
  a[2] = 'o';
  a[3] = 's';
  a[4] = 't';
  a[5] = '\0';

  std::cout << a.origin() << '\n';
}

Boost.MultiArray provides the class boost::multi_array to create arrays. This is the most important class provided. It is defined in boost/multi_array.hpp.

boost::multi_array is a template expecting two parameters: The first parameter is the type of the elements to store in the array. The second parameter determines how many dimensions the array should have.

The second parameter only sets the number of dimensions, not the number of elements in each dimension. Thus, in Example 19.1, a is a one-dimensional array.

The number of elements in a dimension is set at runtime. Example 19.1 uses the global object boost::extents to set dimension sizes. This object is passed to the constructor of a.

An object of type boost::multi_array can be used like a normal C array. Elements are accessed by passing an index to operator[]. Example 19.1 stores five letters and a null character in a – a one-dimensional array with six elements. origin() returns a pointer to the first element. The example uses this pointer to write the word stored in the array – Boost – to standard output.

Unlike containers from the standard library, operator[] checks whether an index is valid. If an index is not valid, the program exits with std::abort(). If you don’t want the validity of indexes to be checked, define the macro BOOST_DISABLE_ASSERTS before you include boost/multi_array.hpp.

#include <boost/multi_array.hpp>
#include <algorithm>
#include <iostream>
#include <cstring>

int main()
{
  boost::multi_array<char, 2> a{boost::extents[2][6]};

  typedef boost::multi_array<char, 2>::array_view<1>::type array_view;
  typedef boost::multi_array_types::index_range range;
  array_view view = a[boost::indices[0][range{0, 5}]];

  std::memcpy(view.origin(), "tsooB", 6);
  std::reverse(view.begin(), view.end());

  std::cout << view.origin() << '\n';

  boost::multi_array<char, 2>::reference subarray = a[1];
  std::memcpy(subarray.origin(), "C++", 4);

  std::cout << subarray.origin() << '\n';
}

Example 19.2 creates a two-dimensional array. The number of elements in the first dimension is set to 2 and for the second dimension set to 6. Think of the array as a table with two rows and six columns.

The first row of the table will contain the word Boost. Since only five letters need to be stored for this word, a view is created which spans exactly five elements of the array.

A view, which is based on the class boost::multi_array::array_view, lets you access a part of an array and treat that part as though it were a separate array.

boost::multi_array::array_view is a template that expects the number of dimensions in the view as a template parameter. In Example 19.2 the number of dimensions for the view is 1. Because the array a has two dimensions, one dimension is ignored. To save the word Boost, a one-dimensional array is sufficient; more dimensions would be confusing.

As with boost::multi_array, the number of dimensions is passed in as a template parameter, and the size of each dimension is set at runtime. However, with boost::multi_array::array_view this isn’t done with boost::extents. Instead it’s done with boost::indices, which is another global object provided by Boost.MultiArray.

As with boost::extents, indexes must be passed to boost::indices. While only numbers may be passed to boost::extents, boost::indices accepts also ranges. These are defined using boost::multi_array_types::index_range.

In Example 19.2, the first parameter passed to boost::indices isn’t a range, it’s the number 0. When a number is passed, you cannot use boost::multi_array_types::index_range. In the example, the view will take the first dimension of a – the one with index 0.

For the second parameter, boost::multi_array_types::index_range is used to define a range. By passing 0 and 5 to the constructor, the first five elements of the first dimension of a are made available. The range starts at index 0 and ends at index 5 – excluding the element at index 5. The sixth element in the first dimension is ignored.

Thus, view is a one-dimensional array consisting of five elements – the first five elements in the first row of a. When view is accessed to copy a string with std::memcpy() and reverse the elements with std::reverse(), this relation doesn’t matter. Once the view is created, it acts like an independent array with five elements.

When operator[] is called on an array of type boost::multi_array, the return value depends on the number of dimensions. In Example 19.1, the operator returns char elements because the array accessed is one dimensional.

In Example 19.2, a is a two-dimensional array. Thus, operator[] returns a subarray rather than a char element. Because the type of the subarray isn’t public, boost::multi_array::reference must be used. This type isn’t identical to boost::multi_array::array_view, even if the subarray behaves like a view. A view must be defined explicitly and can span arbitrary parts of an array, whereas a subarray is automatically returned by operator[] and spans all elements in every dimension.

#include <boost/multi_array.hpp>
#include <algorithm>
#include <iostream>
#include <cstring>

int main()
{
  char c[12] =
  {
    't', 's', 'o', 'o', 'B', '\0',
    'C', '+', '+', '\0', '\0', '\0'
  };

  boost::multi_array_ref<char, 2> a{c, boost::extents[2][6]};

  typedef boost::multi_array<char, 2>::array_view<1>::type array_view;
  typedef boost::multi_array_types::index_range range;
  array_view view = a[boost::indices[0][range{0, 5}]];

  std::reverse(view.begin(), view.end());
  std::cout << view.origin() << '\n';

  boost::multi_array<char, 2>::reference subarray = a[1];
  std::cout << subarray.origin() << '\n';
}

The class boost::multi_array_ref wraps an existing C array. In Example 19.3, a provides the same interface as boost::multi_array, but without allocating memory. With boost::multi_array_ref, a C array – no matter how many dimensions it has – can be treated like a multidimensional array of type boost::multi_array. The C array just needs to be added as an additional parameter to the constructor.

Boost.MultiArray also provides the class boost::const_multi_array_ref, which treats a C array as a constant multidimensional array.