How do I find the length of an array?

Question

Is there a way to find how many values an array has? Detecting whether or not I\'ve reached the end of an array would also work.

Answer 1

Since C++11, some new templates are introduced to help reduce the pain when dealing with array length. All of them are defined in header .

• std::rank::value

  If T is an array type, provides the member constant value equal to the number of dimensions of the array. For any other type, value is 0.

• std::extent::value

  If T is an array type, provides the member constant value equal to the number of elements along the Nth dimension of the array, if N is in [0, std::rank::value). For any other type, or if T is array of unknown bound along its first dimension and N is 0, value is 0.

• std::remove_extent::type

  If T is an array of some type X, provides the member typedef type equal to X, otherwise type is T. Note that if T is a multidimensional array, only the first dimension is removed.

• std::remove_all_extents::type

  If T is a multidimensional array of some type X, provides the member typedef type equal to X, otherwise type is T.

To get the length on any dimension of a multidimential array, decltype could be used to combine with std::extent. For example:

#include 
#include  // std::remove_extent std::remove_all_extents std::rank std::extent

template
constexpr size_t length(T(&)[N]) { return N; }

template
constexpr size_t length2(T(&arr)[N]) { return sizeof(arr) / sizeof(*arr); }

int main()
{
    int a[5][4][3]{{{1,2,3}, {4,5,6}}, { }, {{7,8,9}}};

    // New way
    constexpr auto l1 = std::extent::value;     // 5
    constexpr auto l2 = std::extent::value;  // 4
    constexpr auto l3 = std::extent::value;  // 3
    constexpr auto l4 = std::extent::value;  // 0

    // Mixed way
    constexpr auto la = length(a);
    //constexpr auto lpa = length(*a);  // compile error
    //auto lpa = length(*a);  // get at runtime
    std::remove_extent::type pa;  // get at compile time
    //std::remove_reference::type pa;  // same as above
    constexpr auto lpa = length(pa);
    std::cout << la << ' ' << lpa << '\n';

    // Old way
    constexpr auto la2 = sizeof(a) / sizeof(*a);
    constexpr auto lpa2 = sizeof(*a) / sizeof(**a);
    std::cout << la2 << ' ' << lpa2 << '\n';

    return 0;
}

BTY, to get the total number of elements in a multidimentional array:

constexpr auto l = sizeof(a) / sizeof(std::remove_all_extents::type);

Or put it in a function template:

#include 
#include 
    

template
constexpr size_t len(T &a)
{
    return sizeof(a) / sizeof(typename std::remove_all_extents::type);
}

int main()
{
    int a[5][4][3]{{{1,2,3}, {4,5,6}}, { }, {{7,8,9}}};
    constexpr auto ttt = len(a);
    int i;
    std::cout << ttt << ' ' << len(i) << '\n';
    
    return 0;
}

More examples of how to use them could be found by following the links.