std::ranges::concat_view<Views...>::size - cppreference.com (original) (raw)

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](../../ranges.html#make-unsigned-like-t "cpp/ranges")_` <CT\>(sizes) + ...); },
`_[tuple-transform](../../ranges.html#tuple-transform "cpp/ranges")_` ([ranges::size](../size.html),` ` `_[views](../concat%5Fview.html#views "cpp/ranges/concat view")_` ) ); .

[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 #include #include #include   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>); 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); }

[edit] See also

| | returns an integer equal to the size of a range(customization point object)[edit] | | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | returns a signed integer equal to the size of a range(customization point object)[edit] |