From 514216d54a7ec218d089964dca48fed25b795ea8 Mon Sep 17 00:00:00 2001
From: Egor Tensin <egor@tensin.name>
Date: Thu, 4 Jun 2026 21:38:49 +0200
Subject: upgrade to concepts and string views

---
 include/winapi/utf8.hpp | 24 ++++++++++++++++--------
 src/convert.cpp         | 18 ++++++++----------
 2 files changed, 24 insertions(+), 18 deletions(-)

diff --git a/include/winapi/utf8.hpp b/include/winapi/utf8.hpp
index 2e87f75..0307507 100644
--- a/include/winapi/utf8.hpp
+++ b/include/winapi/utf8.hpp
@@ -10,9 +10,11 @@
 
 #pragma once
 
+#include <concepts>
 #include <cstddef>
 #include <memory>
 #include <string>
+#include <string_view>
 #include <vector>
 
 namespace winapi {
@@ -21,10 +23,8 @@ namespace winapi {
 static_assert(sizeof(wchar_t) == 2, "This is Windows, right?");
 
 /** Convert UTF-8 string to UTF-16. */
-std::wstring widen(const std::string&);
-#ifdef __cpp_lib_char8_t
-std::wstring widen(const std::u8string&);
-#endif
+std::wstring widen(std::string_view);
+std::wstring widen(std::u8string_view);
 
 /**
  * Convert UTF-8 string to UTF-16.
@@ -33,20 +33,24 @@ std::wstring widen(const std::u8string&);
  */
 std::wstring widen(const void* src, std::size_t nb);
 
+template <typename T>
+concept CanBeWidened =
+    std::same_as<T, char> || std::same_as<T, char8_t> || std::same_as<T, unsigned char>;
+
 /**
  * Convert UTF-8 string to UTF-16.
  * \param src Buffer holding a UTF-8 string.
  */
-template <typename T, typename Alloc = std::allocator<T>>
+template <CanBeWidened T, typename Alloc = std::allocator<T>>
 std::wstring widen(const std::vector<T, Alloc>& src) {
     return widen(src.data(), src.size() * sizeof(T));
 }
 
 /** Convert UTF-16 string to UTF-8. */
-std::string narrow(const std::wstring&);
+std::string narrow(std::wstring_view);
 
 /** Convert UTF-16 string to UTF-8. */
-std::string narrow(const std::u16string&);
+std::string narrow(std::u16string_view);
 
 /**
  * Convert UTF-16 string to UTF-8.
@@ -55,11 +59,15 @@ std::string narrow(const std::u16string&);
  */
 std::string narrow(const void* src, std::size_t nb);
 
+template <typename T>
+concept CanBeNarrowed =
+    std::same_as<T, wchar_t> || std::same_as<T, char16_t> || std::same_as<T, unsigned char>;
+
 /**
  * Convert UTF-16 string to UTF-8.
  * \param src Buffer holding a UTF-16 string.
  */
-template <typename T, typename Alloc = std::allocator<T>>
+template <CanBeNarrowed T, typename Alloc = std::allocator<T>>
 std::string narrow(const std::vector<T, Alloc>& src) {
     return narrow(src.data(), src.size() * sizeof(T));
 }
diff --git a/src/convert.cpp b/src/convert.cpp
index 0f96767..675d244 100644
--- a/src/convert.cpp
+++ b/src/convert.cpp
@@ -80,15 +80,13 @@ void verify_output(const std::vector<CharT>& expected, int32_t _actual_size) {
 
 } // namespace
 
-std::wstring widen(const std::string& src) {
-    return widen(src.c_str(), src.size());
+std::wstring widen(std::string_view src) {
+    return widen(src.data(), src.length());
 }
 
-#ifdef __cpp_lib_char8_t
-std::wstring widen(const std::u8string& src) {
-    return widen(src.c_str(), src.size());
+std::wstring widen(std::u8string_view src) {
+    return widen(src.data(), src.length());
 }
-#endif
 
 std::wstring widen(const void* src, std::size_t in_nb) {
     const DWORD flags = MB_ERR_INVALID_CHARS;
@@ -115,13 +113,13 @@ std::wstring widen(const void* src, std::size_t in_nb) {
     return {out.data(), out.size()};
 }
 
-std::string narrow(const std::wstring& src) {
+std::string narrow(std::wstring_view src) {
     static_assert(sizeof(wchar_t) == sizeof(WCHAR), "wchar_t != WCHAR");
-    return narrow(src.c_str(), src.size() * sizeof(std::wstring::value_type));
+    return narrow(src.data(), src.length() * sizeof(std::wstring_view::value_type));
 }
 
-std::string narrow(const std::u16string& src) {
-    return narrow(src.c_str(), src.size() * sizeof(std::u16string::value_type));
+std::string narrow(std::u16string_view src) {
+    return narrow(src.data(), src.length() * sizeof(std::u16string_view::value_type));
 }
 
 std::string narrow(const void* src, std::size_t in_nb) {
-- 
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