add Chapter18

Chapter18/cib/libs/stdx/cached.hpp

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+#pragma once
+
+#include <stdx/compiler.hpp>
+#include <stdx/functional.hpp>
+#include <stdx/type_traits.hpp>
+
+#include <optional>
+#include <type_traits>
+#include <utility>
+
+namespace stdx {
+inline namespace v1 {
+template <typename F> struct cached {
+    using value_type = stdx::remove_cvref_t<std::invoke_result_t<F>>;
+
+    constexpr explicit cached(F const &f) : lazy{f} {}
+    constexpr explicit cached(F &&f) : lazy{std::move(f)} {}
+
+    constexpr auto has_value() const noexcept -> bool {
+        return opt.has_value();
+    }
+    constexpr explicit operator bool() const noexcept {
+        return opt.has_value();
+    }
+
+    constexpr auto value() & LIFETIMEBOUND -> value_type & {
+        populate();
+        return *opt;
+    }
+    constexpr auto value() const & LIFETIMEBOUND -> value_type const & {
+        populate();
+        return *opt;
+    }
+    constexpr auto value() && LIFETIMEBOUND -> value_type && {
+        populate();
+        return *std::move(opt);
+    }
+    constexpr auto value() const && LIFETIMEBOUND -> value_type const && {
+        populate();
+        return *std::move(opt);
+    }
+
+    constexpr auto operator->() const LIFETIMEBOUND->value_type const * {
+        populate();
+        return opt.operator->();
+    }
+    constexpr auto operator->() LIFETIMEBOUND->value_type * {
+        populate();
+        return opt.operator->();
+    }
+
+    constexpr auto operator*() const & LIFETIMEBOUND->decltype(auto) {
+        return value();
+    }
+    constexpr auto operator*() & LIFETIMEBOUND->decltype(auto) {
+        return value();
+    }
+    constexpr auto operator*() const && LIFETIMEBOUND->decltype(auto) {
+        return std::move(*this).value();
+    }
+    constexpr auto operator*() && LIFETIMEBOUND->decltype(auto) {
+        return std::move(*this).value();
+    }
+
+    auto reset() { opt.reset(); }
+    auto refresh() LIFETIMEBOUND -> value_type & {
+        opt.reset();
+        populate();
+        return *opt;
+    }
+
+  private:
+    constexpr auto populate() const {
+        if (not opt.has_value()) {
+            opt.emplace(lazy);
+        }
+    }
+
+    with_result_of<F> lazy;
+    mutable std::optional<value_type> opt{};
+};
+
+template <typename C>
+using cached_value_t = typename stdx::remove_cvref_t<C>::value_type;
+} // namespace v1
+} // namespace stdx