StringUtil: Add IsPrintableCharacter and use it

Add a function that safely returns whether a character is printable
i.e. whether 0x20 <= c <= 0x7e is true.

This is done in several places in our codebase and it's easy to run
into undefined behaviour if the C version defined in <cctype>
is used instead of this one, since its behaviour is undefined
if the character is not representable as an unsigned char.

This fixes MemoryViewWidget.

Source/Core/Common/StringUtil.cpp

@@ -71,7 +71,7 @@ std::string HexDump(const u8* data, size_t size)
       if (row_start + i < size)
       {
         char c = static_cast<char>(data[row_start + i]);
-        out += std::isprint(c, std::locale::classic()) ? c : '.';
+        out += IsPrintableCharacter(c) ? c : '.';
       }
     }
     out += "\n";

Source/Core/Common/StringUtil.h

@@ -222,3 +222,11 @@ std::string ThousandSeparate(I value, int spaces = 0)
   return stream.str();
 #endif
 }
+
+/// Returns whether a character is printable, i.e. whether 0x20 <= c <= 0x7e is true.
+/// Use this instead of calling std::isprint directly to ensure
+/// the C locale is being used and to avoid possibly undefined behaviour.
+inline bool IsPrintableCharacter(char c)
+{
+  return std::isprint(c, std::locale::classic());
+}

Source/Core/Core/Core.cpp

@@ -4,9 +4,9 @@
 
 #include "Core/Core.h"
 
+#include <algorithm>
 #include <atomic>
 #include <cstring>
-#include <locale>
 #include <mutex>
 #include <queue>
 #include <utility>
@@ -31,6 +31,7 @@
 #include "Common/MemoryUtil.h"
 #include "Common/MsgHandler.h"
 #include "Common/ScopeGuard.h"
+#include "Common/StringUtil.h"
 #include "Common/Thread.h"
 #include "Common/Timer.h"
 #include "Common/Version.h"
@@ -160,11 +161,8 @@ void DisplayMessage(std::string message, int time_in_ms)
     return;
 
   // Actually displaying non-ASCII could cause things to go pear-shaped
-  for (const char& c : message)
-  {
-    if (!std::isprint(c, std::locale::classic()))
-      return;
-  }
+  if (!std::all_of(message.begin(), message.end(), IsPrintableCharacter))
+    return;
 
   Host_UpdateTitle(message);
   OSD::AddMessage(std::move(message), time_in_ms);

Source/Core/Core/IOS/ES/Formats.cpp

@@ -9,7 +9,6 @@
 #include <cinttypes>
 #include <cstddef>
 #include <cstring>
-#include <locale>
 #include <map>
 #include <optional>
 #include <string>
@@ -314,11 +313,7 @@ std::string TMDReader::GetGameID() const
   std::memcpy(game_id, m_bytes.data() + offsetof(TMDHeader, title_id) + 4, 4);
   std::memcpy(game_id + 4, m_bytes.data() + offsetof(TMDHeader, group_id), 2);
 
-  const bool all_printable = std::all_of(std::begin(game_id), std::end(game_id), [](char c) {
-    return std::isprint(c, std::locale::classic());
-  });
-
-  if (all_printable)
+  if (std::all_of(std::begin(game_id), std::end(game_id), IsPrintableCharacter))
     return std::string(game_id, sizeof(game_id));
 
   return fmt::format("{:016x}", GetTitleId());
@@ -329,10 +324,7 @@ std::string TMDReader::GetGameTDBID() const
   const u8* begin = m_bytes.data() + offsetof(TMDHeader, title_id) + 4;
   const u8* end = begin + 4;
 
-  const bool all_printable =
-      std::all_of(begin, end, [](char c) { return std::isprint(c, std::locale::classic()); });
-
-  if (all_printable)
+  if (std::all_of(begin, end, IsPrintableCharacter))
     return std::string(begin, end);
 
   return fmt::format("{:016x}", GetTitleId());

Source/Core/Core/IOS/USB/USBV4.cpp

@@ -5,10 +5,11 @@
 #include "Core/IOS/USB/USBV4.h"
 
 #include <algorithm>
-#include <locale>
+#include <functional>
 #include <string>
 
 #include "Common/CommonTypes.h"
+#include "Common/StringUtil.h"
 #include "Common/Swap.h"
 #include "Core/HW/Memmap.h"
 #include "Core/IOS/Device.h"
@@ -75,11 +76,8 @@ V4GetUSStringMessage::V4GetUSStringMessage(Kernel& ios, const IOCtlRequest& ioct
 
 void V4GetUSStringMessage::OnTransferComplete(s32 return_value) const
 {
-  const std::locale& c_locale = std::locale::classic();
   std::string message = Memory::GetString(data_address);
-  std::replace_if(
-      message.begin(), message.end(), [&c_locale](char c) { return !std::isprint(c, c_locale); },
-      '?');
+  std::replace_if(message.begin(), message.end(), std::not_fn(IsPrintableCharacter), '?');
   Memory::CopyToEmu(data_address, message.c_str(), message.size());
   TransferCommand::OnTransferComplete(return_value);
 }

Source/Core/DiscIO/VolumeWad.cpp

@@ -5,7 +5,6 @@
 #include <algorithm>
 #include <cstddef>
 #include <cstring>
-#include <locale>
 #include <map>
 #include <memory>
 #include <optional>
@@ -261,8 +260,7 @@ std::string VolumeWAD::GetMakerID(const Partition& partition) const
     return "00";
 
   // Some weird channels use 0x0000 in place of the MakerID, so we need a check here
-  const std::locale& c_locale = std::locale::classic();
-  if (!std::isprint(temp[0], c_locale) || !std::isprint(temp[1], c_locale))
+  if (!IsPrintableCharacter(temp[0]) || !IsPrintableCharacter(temp[1]))
     return "00";
 
   return DecodeString(temp);

Source/Core/DolphinQt/Debugger/MemoryViewWidget.cpp

@@ -14,6 +14,7 @@
 #include <cctype>
 #include <cmath>
 
+#include "Common/StringUtil.h"
 #include "Core/Core.h"
 #include "Core/HW/AddressSpace.h"
 #include "Core/PowerPC/BreakPoints.h"
@@ -169,8 +170,8 @@ void MemoryViewWidget::Update()
     case Type::ASCII:
       update_values([&accessors](u32 address) {
         const char value = accessors->ReadU8(address);
-        return std::isprint(value) ? QString{QChar::fromLatin1(value)} :
-                                     QString{QChar::fromLatin1('.')};
+        return IsPrintableCharacter(value) ? QString{QChar::fromLatin1(value)} :
+                                             QString{QChar::fromLatin1('.')};
       });
       break;
     case Type::U16: