add Chapter18

Chapter18/cib/libs/flow/log.hpp

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+#pragma once
+
+#include <log/log.hpp>
+
+#include <stdx/ct_string.hpp>
+
+#include <type_traits>
+
+namespace flow {
+struct default_log_spec {
+    using flavor = logging::default_flavor_t;
+    constexpr static auto level = logging::level::TRACE;
+};
+template <stdx::ct_string, typename...>
+constexpr auto log_spec = default_log_spec{};
+
+template <stdx::ct_string Name> struct log_spec_id_t {
+    constexpr static auto ct_name = Name;
+};
+
+template <typename T, typename Fallback = log_spec_id_t<"default">,
+          typename... DummyArgs>
+    requires(sizeof...(DummyArgs) == 0)
+constexpr static auto get_log_spec() {
+    using log_spec_t = decltype(log_spec<T::ct_name, DummyArgs...>);
+    if constexpr (std::is_same_v<log_spec_t, default_log_spec const>) {
+        if constexpr (Fallback::ct_name == stdx::ct_string{"default"}) {
+            return log_spec<Fallback::ct_name, DummyArgs...>;
+        } else {
+            return get_log_spec<Fallback>();
+        }
+    } else {
+        return log_spec_t{};
+    }
+}
+} // namespace flow