Update for Qt-6.6

Qt-6.6 deprecates qAsConst in favour of std::as_const() but the latter
is only available as of C++17. Therefore define a local helper template
instead.

README.md

@@ -156,13 +156,13 @@ cmake --build build --target package
 Qt
 --
 
-This project supports Qt 5.12.x, Qt 5.15.x and Qt 6.5.x (although it may be
-possible to use other versions of Qt) when building the `dukpt-ui` application.
-However, on some platforms it may be necessary to use the `QT_DIR` option (and
-not the `Qt5_DIR` nor `Qt6_DIR` options) or `CMAKE_PREFIX_PATH` option to
-specify the exact Qt installation to be used. For Qt6 it may also be necessary
-for the Qt tools to be available in the executable PATH regardless of the
-`QT_DIR` option.
+This project supports Qt 5.12.x, Qt 5.15.x, Qt 6.5.x and Qt 6.6.x (although it
+may be possible to use other versions of Qt) when building the `dukpt-ui`
+application. However, on some platforms it may be necessary to use the `QT_DIR`
+option (and not the `Qt5_DIR` nor `Qt6_DIR` options) or `CMAKE_PREFIX_PATH`
+option to specify the exact Qt installation to be used. For Qt6 it may also be
+necessary for the Qt tools to be available in the executable PATH regardless of
+the `QT_DIR` option.
 
 If the Qt installation does not provide universal binaries for MacOS, it will
 not be possible to build `dukpt-ui` as a universal binary for MacOS.

ui/validators.cpp

@@ -27,6 +27,17 @@
 
 #include <cctype>
 
+// QString performs a deep copy when begin() is called by a ranged-based for
+// loop, but std::as_const() is only available as of C++17 and qAsConst is
+// deprecated by Qt-6.6. Therefore define a local helper template instead.
+template<class T>
+constexpr std::add_const_t<T>& to_const_ref(T& t) noexcept
+{
+	return t;
+}
+template<class T>
+void to_const_ref(const T&&) = delete;
+
 void DecStringValidator::setMinLength(unsigned int x)
 {
 	if (x != minLength) {
@@ -62,7 +73,7 @@ QValidator::State DecStringValidator::validate(QString& input, int& pos) const
 	}
 
 	// Ensure that decimal string contains only decimal digits
-	for (QChar c : qAsConst(input)) {
+	for (QChar c : to_const_ref(input)) {
 		if (!std::isdigit(c.toLatin1())) {
 			// Non-decimal digit is not allowed
 			return Intermediate;
@@ -80,7 +91,7 @@ QValidator::State HexStringValidator::validate(QString& input, int& pos) const
 	}
 
 	// Ensure that hex string contains only hex digits
-	for (QChar c : qAsConst(input)) {
+	for (QChar c : to_const_ref(input)) {
 		if (!std::isxdigit(c.toLatin1())) {
 			// Non-hex digit is not allowed
 			return Intermediate;