std::ranges::concat_view<Views...>:: size

Returns the number of elements.

Equivalent to return std:: apply
(
[ ] ( auto ... sizes )
{
using CT = ranges:: common_type_t < decltype ( sizes ) ... > ;
return ( make-unsigned-like-t  < CT > ( sizes ) + ... ) ;
} ,
tuple-transform  ( ranges:: size , views_  )
) ; .

Return value

As described above.

Complexity

Constant.

Notes

The complexity of concat_view is constant time (even though in some cases it is a linear function of the number of ranges it concatenates which is a statically known parameter of this view) because time complexity as required by the ranges concepts are formally expressed with respect to the total number of elements (the size) of a given range, and not to the statically known parameters of that range.

Example

#include <cassert>
#include <forward_list>
#include <list>
#include <ranges>
 
int main()
{
    constexpr static auto a = {1, 2};
    constexpr static auto b = {1, 2, 3};
    constexpr static auto c = {1, 2, 3, 4};
 
    constexpr auto con{std::views::concat(a, b, c)};
    static_assert(std::ranges::sized_range<decltype(con)>);
    static_assert(con.size() == 2 + 3 + 4);
 
    std::forward_list d = b;
    static_assert(not std::ranges::sized_range<std::forward_list<int>>);
    const auto cat{std::views::concat(b, c, d)};
    static_assert(not std::ranges::sized_range<decltype(cat)>);
//  auto x = cat.size(); // error: cat is not sized_range because of d
 
    std::list e = c;
    const auto dog{std::views::concat(a, b, e)};
    static_assert(std::ranges::sized_range<decltype(dog)>);
    assert(dog.size() == 2 + 3 + 4);
}

See also