Add held-buffer converter helpers for hot paths.

src/types/convert.cpp

@@ -27,11 +27,38 @@ static const WORD leftShiftScanCode = 0x2A;
 // - The UTF-16 wide string.
 // - NOTE: Throws suitable HRESULT errors from memory allocation, safe math, or MultiByteToWideChar failures.
 [[nodiscard]] std::wstring ConvertToW(const UINT codePage, const std::string_view source)
+{
+    // Make a buffer on behalf of the caller.
+    std::wstring out;
+
+    // Call the other form of the function.
+    ConvertToW(codePage, source, out);
+
+    // Return as a wstring
+    return out;
+}
+
+// Routine Description:
+// - Takes a multibyte string, allocates the appropriate amount of memory for the conversion, performs the conversion,
+//   and returns the Unicode UTF-16 result in the smart pointer (and the length).
+// - NOTE: This form exists so a frequent caller with a hot path can cache their string
+//   buffer between calls instead of letting it get new/deleted in a tight loop.
+// Arguments:
+// - codepage - Windows Code Page representing the multibyte source text
+// - source - View of multibyte characters of source text
+// - outBuffer - The buffer to fill with converted wide string data.
+// Return Value:
+// - The UTF-16 wide string.
+// - NOTE: Throws suitable HRESULT errors from memory allocation, safe math, or MultiByteToWideChar failures.
+[[nodiscard]] void ConvertToW(const UINT codePage,
+                              const std::string_view source,
+                              std::wstring& outBuffer)
 {
     // If there's nothing to convert, bail early.
     if (source.empty())
     {
-        return {};
+        outBuffer.clear();
+        return;
     }
 
     int iSource; // convert to int because Mb2Wc requires it.
@@ -52,11 +79,28 @@ static const WORD leftShiftScanCode = 0x2A;
     THROW_IF_FAILED(IntToSizeT(iTarget, &cchNeeded));
 
     // Allocate ourselves some space
-    std::wstring out;
-    out.resize(cchNeeded);
+    outBuffer.resize(cchNeeded);
 
     // Attempt conversion for real.
-    THROW_LAST_ERROR_IF_AND_IGNORE_BAD_GLE(0 == MultiByteToWideChar(codePage, 0, source.data(), iSource, out.data(), iTarget));
+    THROW_LAST_ERROR_IF_AND_IGNORE_BAD_GLE(0 == MultiByteToWideChar(codePage, 0, source.data(), iSource, outBuffer.data(), iTarget));
+}
+
+// Routine Description:
+// - Takes a wide string, allocates the appropriate amount of memory for the conversion, performs the conversion,
+//   and returns the Multibyte result
+// Arguments:
+// - codepage - Windows Code Page representing the multibyte destination text
+// - source - Unicode (UTF-16) characters of source text
+// Return Value:
+// - The multibyte string encoded in the given codepage
+// - NOTE: Throws suitable HRESULT errors from memory allocation, safe math, or MultiByteToWideChar failures.
+[[nodiscard]] std::string ConvertToA(const UINT codepage, const std::wstring_view source)
+{
+    // Make a buffer on behalf of the caller.
+    std::string out;
+
+    // Call the other form of the function.
+    ConvertToA(codepage, source, out);
 
     // Return as a string
     return out;
@@ -65,18 +109,24 @@ static const WORD leftShiftScanCode = 0x2A;
 // Routine Description:
 // - Takes a wide string, allocates the appropriate amount of memory for the conversion, performs the conversion,
 //   and returns the Multibyte result
+// - NOTE: This form exists so a frequent caller with a hot path can cache their string
+//   buffer between calls instead of letting it get new/deleted in a tight loop.
 // Arguments:
 // - codepage - Windows Code Page representing the multibyte destination text
 // - source - Unicode (UTF-16) characters of source text
+// - outBuffer - The buffer to fill with converted string data.
 // Return Value:
 // - The multibyte string encoded in the given codepage
 // - NOTE: Throws suitable HRESULT errors from memory allocation, safe math, or MultiByteToWideChar failures.
-[[nodiscard]] std::string ConvertToA(const UINT codepage, const std::wstring_view source)
+[[nodiscard]] void ConvertToA(const UINT codepage,
+                              const std::wstring_view source,
+                              std::string& outBuffer)
 {
     // If there's nothing to convert, bail early.
     if (source.empty())
     {
-        return {};
+        outBuffer.clear();
+        return;
     }
 
     int iSource; // convert to int because Wc2Mb requires it.
@@ -93,17 +143,13 @@ static const WORD leftShiftScanCode = 0x2A;
     THROW_IF_FAILED(IntToSizeT(iTarget, &cchNeeded));
 
     // Allocate ourselves some space
-    std::string out;
-    out.resize(cchNeeded);
+    outBuffer.resize(cchNeeded);
 
     // Attempt conversion for real.
     // clang-format off
 #pragma prefast(suppress: __WARNING_W2A_BEST_FIT, "WC_NO_BEST_FIT_CHARS doesn't work in many codepages. Retain old behavior.")
     // clang-format on
-    THROW_LAST_ERROR_IF(0 == WideCharToMultiByte(codepage, 0, source.data(), iSource, out.data(), iTarget, nullptr, nullptr));
-
-    // Return as a string
-    return out;
+    THROW_LAST_ERROR_IF(0 == WideCharToMultiByte(codepage, 0, source.data(), iSource, outBuffer.data(), iTarget, nullptr, nullptr));
 }
 
 // Routine Description:

src/types/inc/convert.hpp

@@ -34,6 +34,17 @@ enum class CodepointWidth : BYTE
 [[nodiscard]] std::string ConvertToA(const UINT codepage,
                                      const std::wstring_view source);
 
+// These out-param forms of the functions exist so a frequent caller on a hot
+// codepath can hold their string buffer between calls instead of
+// letting us allocate one for it to be thrown away shortly after.
+[[nodiscard]] void ConvertToW(const UINT codepage,
+                              const std::string_view source,
+                              std::wstring& outBuffer);
+
+[[nodiscard]] void ConvertToA(const UINT codepage,
+                              const std::wstring_view source,
+                              std::string& outBuffer);
+
 [[nodiscard]] size_t GetALengthFromW(const UINT codepage,
                                      const std::wstring_view source);