What is the type of a boost::extent object after providing dimensions?

Question

Say I have

#include <boost/multi_array.hpp>
using intArray3D = boost::multi_array<int, 3>;

and I want to create a bunch of intArray3Ds with the same shape:

auto my_shape = boost::extents[3][4][5];
intArray3D xs(my_shape), ys(my_shape), zs(my_shape);

It's easy enough to use auto to assign boost::extents[3][4][5] into a variable, but how can I concretely figure out the underlying type?

Answer 1

Most importantly,

1. you don't have to know
2. you don't have to use extents either

Many things are acceptible as long they satisfy the documented criteria:

The Collection concept is documented in that link

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#include <boost/multi_array.hpp>
#include <iostream>
using intArray3D = boost::multi_array<int, 3>;

void dump_shape(intArray3D const& arr) {
    for (unsigned dim = 0; dim < arr.dimensionality; ++dim)
        std::cout << arr.shape()[dim] << " ";
    std::cout << "\n";
}

int main() {
    {
        auto my_shape = boost::extents[3][4][5];
        intArray3D xs(my_shape), ys(my_shape), zs(my_shape);
        dump_shape(xs); dump_shape(ys); dump_shape(zs);
    }

    {
        std::array<int, 3> my_shape { 3, 4, 5 };
        intArray3D xs(my_shape), ys(my_shape), zs(my_shape);
        dump_shape(xs); dump_shape(ys); dump_shape(zs);
    }

    {
        std::vector<int> my_shape { 3, 4, 5 };
        intArray3D xs(my_shape), ys(my_shape), zs(my_shape);
        dump_shape(xs); dump_shape(ys); dump_shape(zs);
    }

}

Prints

3 4 5 
3 4 5 
3 4 5 
3 4 5 
3 4 5 
3 4 5 
3 4 5 
3 4 5 
3 4 5 

Answer 2

The documentation mentions:

template gen_type<Ranges>::type

• This type generator is used to specify the result of Ranges chained calls to extent_gen::operator[].

where gen_type is a member of boost::multi_array_types::extent_gen (and boost::multi_array_types::extent_gen is also the type of the global helper object boost::extents).

You can also see that constructors which accept a set of extents are specified in this way (at least for purposes of public documentation). For example,

namespace boost {

template <typename ValueType, 
          std::size_t NumDims, 
          typename Allocator = std::allocator<ValueType> >
class multi_array {

...

  typedef multi_array_types::extent_gen         extent_gen;

...

  explicit multi_array(extent_gen::gen_type<NumDims>::type ranges,
                       const storage_order_type& store = c_storage_order(),
                       const Allocator& alloc = Allocator());

So you could rewrite that line of your code without using auto as:

boost::multi_array_types::extent_gen::gen_type<3>::type my_shape =
    boost::extents[3][4][5];

This is a bit silly for a local variable, but maybe you want to store a set of extents in a class or something like that. If so, this is the way to do it according to the officially documented interface.

(As noted in comments, the actual type this typedef resolves to involves boost::internal::, but you should never use anything from an "internal" namespace in your code, because that is subject to change in future versions.)