Finish unit test for CFX_ByteStringC class.

core/include/fxcrt/fx_string.h

 // Copyright 2014 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
 #ifndef _FX_STRING_H_
 #define _FX_STRING_H_
 class CFX_ByteStringC;
 class CFX_ByteString;
 class CFX_WideStringC;
 class CFX_WideString;
 struct CFX_CharMap;
 class CFX_BinaryBuf;
 typedef int FX_STRSIZE;
 class CFX_ByteStringL;
 class CFX_WideStringL;
+
+// An immutable string with caller-provided storage which must outlive the
+// string itself.
 class CFX_ByteStringC : public CFX_Object
 {
 public:
 
     CFX_ByteStringC()
     {
         m_Ptr = NULL;
         m_Length = 0;
     }
 
     CFX_ByteStringC(FX_LPCBYTE ptr, FX_STRSIZE size)
     {
         m_Ptr = ptr;
         m_Length = size;
     }
 
     CFX_ByteStringC(FX_LPCSTR ptr)
     {
         m_Ptr = (FX_LPCBYTE)ptr;
         m_Length = ptr ? (FX_STRSIZE)FXSYS_strlen(ptr) : 0;
     }
 
+    // |ch| must be an lvalue that outlives the the CFX_ByteStringC. However,
+    // the use of char rvalues are not caught at compile time.  They are
+    // implicitly promoted to CFX_ByteString (see below) and then the
+    // CFX_ByteStringC is constructed from the CFX_ByteString via the alternate
+    // constructor below. The CFX_ByteString then typically goes out of scope
+    // and |m_Ptr| may be left pointing to invalid memory. Beware.
+    // TODO(tsepez): Mark single-argument string constructors as explicit.
     CFX_ByteStringC(FX_CHAR& ch)
     {
         m_Ptr = (FX_LPCBYTE)&ch;
         m_Length = 1;
     }
 
     CFX_ByteStringC(FX_LPCSTR ptr, FX_STRSIZE len)
     {
         m_Ptr = (FX_LPCBYTE)ptr;
         if (len == -1) {
             m_Length = (FX_STRSIZE)FXSYS_strlen(ptr);
         } else {
             m_Length = len;
         }
     }
 
     CFX_ByteStringC(const CFX_ByteStringC& src)
     {
         m_Ptr = src.m_Ptr;
         m_Length = src.m_Length;
     }
 
     CFX_ByteStringC(const CFX_ByteString& src);
 
     CFX_ByteStringC& operator = (FX_LPCSTR src)
     {
         m_Ptr = (FX_LPCBYTE)src;
-        m_Length = (FX_STRSIZE)FXSYS_strlen(src);
+        m_Length = m_Ptr ? (FX_STRSIZE)FXSYS_strlen(src) : 0;
         return *this;
     }
 
     CFX_ByteStringC& operator = (const CFX_ByteStringC& src)
     {
         m_Ptr = src.m_Ptr;
         m_Length = src.m_Length;
         return *this;
     }
 
@@ skipping 773 matching lines @@
 CFX_ByteString  FX_UTF8Encode(FX_LPCWSTR pwsStr, FX_STRSIZE len);
 inline CFX_ByteString   FX_UTF8Encode(FX_WSTR wsStr)
 {
     return FX_UTF8Encode(wsStr.GetPtr(), wsStr.GetLength());
 }
 inline CFX_ByteString   FX_UTF8Encode(const CFX_WideString &wsStr)
 {
     return FX_UTF8Encode(wsStr.c_str(), wsStr.GetLength());
 }
 #endif