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c++multidimensional-arrayc++11variadic-templatesbuilt-in
Ricky65
Ricky65

Reputation: 1687

Size of built-in multidimensional array using variadic template function

In C++11 it is possible to create a function which returns the size (number of elements) of a built-in one dimensional array at compile time using constexpr. Example below:

template <typename T, std::size_t N>
constexpr std::size_t size(T (&array)[N])
{
     return N;
}

This is a superior alternative to ARRAY_SIZE and similar macros imo.

However, this will only return the size of the most significant dimension of a built-in multidimensional array.

I use the following function for determining the size of a built-in two dimensional array:

template <typename T, std::size_t N, std::size_t N2>
constexpr std::size_t size(T (&array)[N][N2])
{
     return N * N2;
}

Ideally, it would be very useful to have a function that returns the size of a built-in array with an arbitrary number of dimensions. I thought variadic templates may help but I couldn't see a way of unpacking the template paramters as only one argument is passed. Is such a function possible?

Thanks in advance.

Upvotes: 4

Views: 1141

Answers (3)

Johannes Schaub - litb
Johannes Schaub - litb

Reputation: 506897

template<typename T> constexpr int size(T const&) { 
  return 1; 
}

template<typename T, int N> constexpr int size(T const (&a)[N]) { 
  return N * size(a[0]); 
}

Upvotes: 3

kennytm
kennytm

Reputation: 523214

#include <type_traits>
#include <cstdlib>

template <typename T>
constexpr size_t size(const T&) noexcept
{
    return sizeof(T)/sizeof(typename std::remove_all_extents<T>::type);
}

Example:

#include <cstdio>
int main()
{
    int a[3][4][7][12];
    char f[6];

    printf("%lu == %ld ?\n", size(a), 3*4*7*12);
    printf("%lu == %ld ?\n", size(f), 6);

    return 0;
}

Upvotes: 5

Potatoswatter
Potatoswatter

Reputation: 137780

You're looking for std::extent. C++11 §20.9.5:

template <class T, unsigned I = 0> struct extent;

If T is not an array type, or if it has rank less than or equal to I, or if I is 0 and T has type “array of unknown bound of U”, then 0; otherwise, the bound (8.3.4) of the I’th dimension of T, where indexing of I is zero-based.

Usage, also from the Standard, prefix extent with std:: as needed:

assert((extent<int[2][4], 1>::value) == 4);

You should also probably use this to replace your custom size function.

Edit: Oops, now I read to the end of the question :vP . You also need std::remove_extent.

template< typename multi_array, bool = std::is_array< multi_array >::value >
struct total_extent;

template< typename multi_array >
struct total_extent< multi_array, false > {
    enum { value = 1 };
};

template< typename multi_array >
struct total_extent< multi_array, true > {
    enum {
        value = std::extent< multi_array >::value
              * total_extent< typename std::remove_extent< multi_array >
                              ::type >::value
    };
};

Upvotes: 2

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