feat: add utf8 format

- move utf8 stream and format patch from string to patch namespace.
- add ordinay format alias and utf8 format in our format patch.
- add char8_t and char inter-cast in string reinterpret namespace.
- fix bug of utf8 formatter.
- add test for utf8 format.

src/yycc/patch/format.hpp

@@ -0,0 +1,111 @@
+/**
+ * @file
+ * @brief The patch for \c std::format to allow UTF8 string as arguments.
+ * @details
+ * As we known, \c std::format only allow \c char and \c wchar_t as its char type in C++ 23 currently.
+ * So it is impossible to use UTF8 string for std::format, both format string and argument.
+ * This namespace give a patch for this shortcoming.
+ * By including this file directly, you will have abilities that use UTF8 string as argument in \c std::format with \c char char type.
+*/
+#pragma once
+#include "../string/reinterpret.hpp"
+#include <format>
+#include <string>
+#include <string_view>
+
+#define NS_YYCC_STRING_REINTERPRET ::yycc::string::reinterpret
+
+#pragma region Utf8 Format
+
+namespace yycc::patch::format {
+
+    // TODO: order all use of std::format redirect to this function.
+
+    template<class... Args>
+    std::string format(std::format_string<Args...> fmt, Args&&... args) {
+        return std::vformat(fmt.get(), std::make_format_args(args...));
+    }
+
+    template<class... Args>
+    std::u8string format(const std::u8string_view& fmt, Args&&... args) {
+        return NS_YYCC_STRING_REINTERPRET::as_utf8(
+            std::vformat(NS_YYCC_STRING_REINTERPRET::as_ordinary_view(fmt), std::make_format_args(args...)));
+    }
+
+} // namespace yycc::patch::format
+
+#pragma endregion
+
+#pragma region Utf8 Formatter
+
+// Add std::formatter specialization for "char8_t"
+template<>
+struct std::formatter<char8_t, char> {
+    constexpr auto parse(auto& ctx) { return underlying_formatter.parse(ctx); }
+    auto format(const char8_t& str, auto& ctx) const { return underlying_formatter.format(NS_YYCC_STRING_REINTERPRET::as_ordinary(str), ctx); }
+
+private:
+    std::formatter<char, char> underlying_formatter{};
+};
+
+// Add std::formatter specialization for "char8_t*"
+template<>
+struct std::formatter<char8_t*, char> {
+    constexpr auto parse(auto& ctx) { return underlying_formatter.parse(ctx); }
+    auto format(char8_t* str, auto& ctx) const { return underlying_formatter.format(NS_YYCC_STRING_REINTERPRET::as_ordinary(str), ctx); }
+
+private:
+    std::formatter<const char*, char> underlying_formatter{};
+};
+
+// Add std::formatter specialization for "const char8_t*"
+template<>
+struct std::formatter<const char8_t*, char> {
+    constexpr auto parse(auto& ctx) { return underlying_formatter.parse(ctx); }
+    auto format(const char8_t* str, auto& ctx) const {
+        return underlying_formatter.format(NS_YYCC_STRING_REINTERPRET::as_ordinary(str), ctx);
+    }
+
+private:
+    std::formatter<const char*, char> underlying_formatter{};
+};
+
+// Add std::formatter specialization for "char8_t[N]"
+template<std::size_t N>
+struct std::formatter<char8_t[N], char> {
+    constexpr auto parse(auto& ctx) { return underlying_formatter.parse(ctx); }
+    auto format(const char8_t (&str)[N], auto& ctx) const {
+        return underlying_formatter.format(std::basic_string_view<char>(NS_YYCC_STRING_REINTERPRET::as_ordinary(str), N - 1), ctx);
+    }
+
+private:
+    std::formatter<std::basic_string_view<char>, char> underlying_formatter{};
+};
+
+// Add std::formatter specialization for "std::u8string"
+template<class Traits, class Alloc>
+struct std::formatter<std::basic_string<char8_t, Traits, Alloc>, char> {
+    constexpr auto parse(auto& ctx) { return underlying_formatter.parse(ctx); }
+    auto format(const std::u8string& str, auto& ctx) const {
+        return underlying_formatter.format(NS_YYCC_STRING_REINTERPRET::as_ordinary_view(str), ctx);
+    }
+
+private:
+    std::formatter<std::basic_string_view<char>, char> underlying_formatter{};
+};
+
+// Add std::formatter specialization for "std::u8string_view"
+template<class Traits>
+struct std::formatter<std::basic_string_view<char8_t, Traits>, char> {
+    constexpr auto parse(auto& ctx) { return underlying_formatter.parse(ctx); }
+    auto format(std::u8string_view sv, auto& ctx) const {
+        return underlying_formatter.format(NS_YYCC_STRING_REINTERPRET::as_ordinary_view(sv), ctx);
+    }
+
+private:
+    std::formatter<std::basic_string_view<char>, char> underlying_formatter{};
+};
+
+#pragma endregion
+
+#undef NS_YYCC_STRING_REINTERPRET

src/yycc/patch/stream.cpp

@@ -0,0 +1,23 @@
+#include "stream.hpp"
+#include "../string/reinterpret.hpp"
+
+#define REINTERPRET ::yycc::string::reinterpret
+
+namespace yycc::patch::stream {
+
+    std::ostream& operator<<(std::ostream& os, const std::u8string_view& u8str) {
+        os << REINTERPRET::as_ordinary_view(u8str);
+        return os;
+    }
+
+    std::ostream& operator<<(std::ostream& os, const char8_t* u8str) {
+        os << REINTERPRET::as_ordinary(u8str);
+        return os;
+    }
+
+    std::ostream& operator<<(std::ostream& os, char8_t u8chr) {
+        os << static_cast<char>(u8chr);
+        return os;
+    }
+
+}

src/yycc/patch/stream.hpp

@@ -0,0 +1,17 @@
+#pragma once
+#include <ostream>
+#include <string_view>
+
+/**
+ * @brief This namespace add UTF8 support for \c std::ostream.
+ * @details
+ * The operator overloads written in this namespace will give \c std::ostream ability to write UTF8 string and its char.
+ * For using this feature, please directly use <TT>using namespace yycc::patch:stream;</TT> to import this namespace.
+ */
+namespace yycc::patch::stream {
+
+    std::ostream& operator<<(std::ostream& os, const std::u8string_view& u8str);
+    std::ostream& operator<<(std::ostream& os, const char8_t* u8str);
+    std::ostream& operator<<(std::ostream& os, char8_t u8chr);
+
+}