add Chapter18

Chapter18/cib/libs/stdx/iterator.hpp

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+#pragma once
+
+#include <stdx/type_traits.hpp>
+
+#include <array>
+#include <cstddef>
+
+namespace stdx {
+inline namespace v1 {
+namespace detail {
+template <typename T> struct ct_capacity_fail {
+    static_assert(always_false_v<stdx::remove_cvref_t<T>>,
+                  "Type does not support compile-time capacity");
+};
+} // namespace detail
+
+template <typename T>
+constexpr auto ct_capacity_v = detail::ct_capacity_fail<T>{};
+
+template <typename T, std::size_t N>
+constexpr auto ct_capacity_v<std::array<T, N>> = N;
+
+template <typename T> constexpr auto ct_capacity_v<T const> = ct_capacity_v<T>;
+
+template <typename T> constexpr auto ct_capacity(T &&) -> std::size_t {
+    return ct_capacity_v<remove_cvref_t<T>>;
+}
+} // namespace v1
+} // namespace stdx
+
+#if __has_include(<span>)
+#include <span>
+
+#if __cpp_lib_span >= 202002L
+namespace stdx {
+inline namespace v1 {
+template <typename T, std::size_t N>
+constexpr auto ct_capacity_v<std::span<T, N>> = N;
+
+template <typename T>
+constexpr auto ct_capacity_v<std::span<T, std::dynamic_extent>> =
+    detail::ct_capacity_fail<std::span<T, std::dynamic_extent>>{};
+} // namespace v1
+} // namespace stdx
+
+#endif
+#endif