std::ranges::concat_view<Views...>::size
requires(sized_range<Views> && ...);
requires(sized_range<const Views> && ...);
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_ ) );
.
Contents
[edit] Return value
As described above.
[edit] Complexity
Constant.
[edit] 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.
[edit] Example
The preliminary version can be checked out on Compiler Explorer.
#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); }