cpp/meta/define static array: Difference between revisions

template< ranges::input_range R >
consteval std::span<const ranges::range_value_t<R>> define_static_array( R&& r );

Promotes array to static storage.

Let N be static_cast<std::size_t>(ranges::size(r)) if ranges::size(r) is a constant expression, or std::dynamic_extent otherwise. The effect is equivalent to:

using T = ranges::range_value_t<R>;
std::meta::info array = std::meta::reflect_constant_array(r);
if (std::meta::is_array_type(std::meta::type_of(array)))
    return std::span<const T, N>(std::meta::extract<const T*>(array),
                                 std::meta::extent(std::meta::type_of(array)));
else
    return std::span<const T, N>(static_cast<const T*>(nullptr), 0);

Parameters

Return value

If the size of r is not zero, a span constructed from an array of type const T[N] (where T is ranges::range_value_t<R> and N is the size of r). Each element of the array is initialized from the value or object represented by std::meta::reflect_constant(static_cast<T>(*it)), where it is an iterator to the corresponding element of r.

Otherwise (the size of r is zero), an empty span.

Notes

As a template parameter object, the resulting array object (if present) has static storage duration. Ranges with template-argument-equivalent contents correspond to the same array object.

The array object is a potentially non-unique object.

Example

#include <algorithm>
#include <meta>
#include <print>
#include <vector>

consteval auto precompute_angles(int size)
{
    std::vector<double> angles(size);
    std::ranges::generate(angles, [&, i{0}] mutable { return 360.0 / size * i++; });
    return angles;
}

consteval auto precompute_angles_arr(int size)
{
    std::vector<double> angles = precompute_angles(size);
    return std::define_static_array(angles);
}

int main()
{
    // auto angles = precompute_angles(7);
    //     error: call to consteval function ‘precompute_angles(7)’ is not a constant
    //            expression because it refers to a result of ‘operator new’

    auto angles = precompute_angles_arr(7);
    for (double angle : angles)
        std::print("{:.1f} ", angle);

    std::println();
}

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See also