Add WTF::String::emptyString16Bit

Source/wtf/text/WTFString.cpp

 /*
  * (C) 1999 Lars Knoll (knoll@kde.org)
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2010, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2007-2009 Torch Mobile, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
@@ skipping 869 matching lines @@
     String result = String::createUninitialized(length, destination);
 
     copyLCharsFromUCharSource(destination, source, length);
 
     return result;
 }
 
 String String::make16BitFrom8BitSource(const LChar* source, size_t length)
 {
     if (!length)
-        return emptyString();
+        return emptyString(false);
 
     UChar* destination;
     String result = String::createUninitialized(length, destination);
 
     StringImpl::copyChars(destination, source, length);
 
     return result;
 }
 
 String String::fromUTF8(const LChar* stringStart, size_t length)
@@ skipping 312 matching lines @@
     // FIXME: This will return ok even when the string fits into a double but not a float.
     return static_cast<float>(toDoubleType<LChar, AllowTrailingJunk>(data, length, 0, parsedLength));
 }
 
 float charactersToFloat(const UChar* data, size_t length, size_t& parsedLength)
 {
     // FIXME: This will return ok even when the string fits into a double but not a float.
     return static_cast<float>(toDoubleType<UChar, AllowTrailingJunk>(data, length, 0, parsedLength));
 }
 
-const String& emptyString()
+const String& emptyString(bool is8Bit)
 {
-    DEFINE_STATIC_LOCAL(String, emptyString, (StringImpl::empty()));
-    return emptyString;
+    DEFINE_STATIC_LOCAL(String, empty8BitString, (StringImpl::empty()));
+    DEFINE_STATIC_LOCAL(String, empty16BitString, (StringImpl::empty(false)));
+    return is8Bit ? empty8BitString : empty16BitString;

[Mikhail, 2014/09/22 10:26:18]

I'm afraid this might affect the performance, what about

const String& emptyString() {}

and

enum Force16Bit { Force16Bit };
const String& emptyString(Force16Bit) {}

 }
 
 } // namespace WTF
 
 #ifndef NDEBUG
 // For use in the debugger
 String* string(const char*);
 Vector<char> asciiDebug(StringImpl* impl);
 Vector<char> asciiDebug(String& string);
 
@@ skipping 29 matching lines @@
     buffer.append('\0');
     return buffer;
 }
 
 Vector<char> asciiDebug(String& string)
 {
     return asciiDebug(string.impl());
 }
 
 #endif