QStringEncoder/Decoder: add missing noexcept

The interesting part is what cannot be noexcept:

- nameForEncoding() and ctor from Encoding: because they don't handle
  all valid values of type Encoding, so have a narrow contract

- encodingForHtml(): because it allocates memory (→ QTBUG-101046)

Change-Id: I30cdc19a32537be047e43955e3337e4d6ccc363f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/text/qstringconverter.cpp

@@ -1699,7 +1699,7 @@ static bool nameMatch(const char *a, const char *b)
 /*!
     \internal
 */
-QStringConverter::QStringConverter(const char *name, Flags f)
+QStringConverter::QStringConverter(const char *name, Flags f) noexcept
     : iface(nullptr), state(f)
 {
     auto e = encodingForName(name);
@@ -1745,7 +1745,7 @@ QStringConverter::QStringConverter(const char *name, Flags f)
     Returns an optional encoding for \a name. The optional is empty if the name could
     not get converted to a valid encoding.
 */
-std::optional<QStringConverter::Encoding> QStringConverter::encodingForName(const char *name)
+std::optional<QStringConverter::Encoding> QStringConverter::encodingForName(const char *name) noexcept
 {
     for (int i = 0; i < LastEncoding + 1; ++i) {
         if (nameMatch(encodingInterfaces[i].name, name))
@@ -1763,7 +1763,8 @@ std::optional<QStringConverter::Encoding> QStringConverter::encodingForName(cons
 
    The returned optional is empty, if the encoding is unclear.
  */
-std::optional<QStringConverter::Encoding> QStringConverter::encodingForData(QByteArrayView data, char16_t expectedFirstCharacter)
+std::optional<QStringConverter::Encoding>
+QStringConverter::encodingForData(QByteArrayView data, char16_t expectedFirstCharacter) noexcept
 {
     // someone set us up the BOM?
     qsizetype arraySize = data.size();

src/corelib/text/qstringconverter.h

@@ -58,17 +58,17 @@ QT_BEGIN_NAMESPACE
 class QStringEncoder : public QStringConverter
 {
 protected:
-    constexpr explicit QStringEncoder(const Interface *i)
+    constexpr explicit QStringEncoder(const Interface *i) noexcept
         : QStringConverter(i)
     {}
 public:
-    constexpr QStringEncoder()
+    constexpr QStringEncoder() noexcept
         : QStringConverter()
     {}
     constexpr explicit QStringEncoder(Encoding encoding, Flags flags = Flag::Default)
         : QStringConverter(encoding, flags)
     {}
-    explicit QStringEncoder(const char *name, Flags flags = Flag::Default)
+    explicit QStringEncoder(const char *name, Flags flags = Flag::Default) noexcept
         : QStringConverter(name, flags)
     {}
 
@@ -116,17 +116,17 @@ private:
 class QStringDecoder : public QStringConverter
 {
 protected:
-    constexpr explicit QStringDecoder(const Interface *i)
+    constexpr explicit QStringDecoder(const Interface *i) noexcept
         : QStringConverter(i)
     {}
 public:
     constexpr explicit QStringDecoder(Encoding encoding, Flags flags = Flag::Default)
         : QStringConverter(encoding, flags)
     {}
-    constexpr QStringDecoder()
+    constexpr QStringDecoder() noexcept
         : QStringConverter()
     {}
-    explicit QStringDecoder(const char *name, Flags f = Flag::Default)
+    explicit QStringDecoder(const char *name, Flags f = Flag::Default) noexcept
         : QStringConverter(name, f)
     {}
 

src/corelib/text/qstringconverter_base.h

@@ -72,7 +72,7 @@ public:
     Q_DECLARE_FLAGS(Flags, Flag)
 
     struct State {
-        constexpr State(Flags f = Flag::Default)
+        constexpr State(Flags f = Flag::Default) noexcept
             : flags(f), state_data{0, 0, 0, 0} {}
         ~State() { clear(); }
         State(State &&other) noexcept
@@ -155,33 +155,35 @@ protected:
         LengthFn fromUtf16Len = nullptr;
     };
 
-    constexpr QStringConverter()
+    constexpr QStringConverter() noexcept
         : iface(nullptr)
     {}
     constexpr explicit QStringConverter(Encoding encoding, Flags f)
         : iface(&encodingInterfaces[int(encoding)]), state(f)
     {}
-    constexpr explicit QStringConverter(const Interface *i)
+    constexpr explicit QStringConverter(const Interface *i) noexcept
         : iface(i)
     {}
-    Q_CORE_EXPORT explicit QStringConverter(const char *name, Flags f);
+    Q_CORE_EXPORT explicit QStringConverter(const char *name, Flags f) noexcept;
 
 
 public:
-    bool isValid() const { return iface != nullptr; }
 
-    void resetState()
+    bool isValid() const noexcept { return iface != nullptr; }
+
+    void resetState() noexcept
     {
         state.clear();
     }
-    bool hasError() const { return state.invalidChars != 0; }
+    bool hasError() const noexcept { return state.invalidChars != 0; }
 
-    const char *name() const
+    const char *name() const noexcept
     { return isValid() ? iface->name : nullptr; }
 
-    Q_CORE_EXPORT static std::optional<Encoding> encodingForName(const char *name);
+    Q_CORE_EXPORT static std::optional<Encoding> encodingForName(const char *name) noexcept;
     Q_CORE_EXPORT static const char *nameForEncoding(Encoding e);
-    Q_CORE_EXPORT static std::optional<Encoding> encodingForData(QByteArrayView data, char16_t expectedFirstCharacter = 0);
+    Q_CORE_EXPORT static std::optional<Encoding>
+    encodingForData(QByteArrayView data, char16_t expectedFirstCharacter = 0) noexcept;
     Q_CORE_EXPORT static std::optional<Encoding> encodingForHtml(QByteArrayView data);
 
 protected: