Make CFX_{Byte,Wide}String::Remove() no-touch if possible

core/fxcrt/fx_basic_bstring_unittest.cpp

 // 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.
 
 #include "core/fxcrt/include/fx_string.h"
 #include "testing/fx_string_testhelpers.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 TEST(fxcrt, ByteStringOperatorSubscript) {
   // CFX_ByteString includes the NUL terminator for non-empty strings.
@@ skipping 315 matching lines @@
   freed.Remove('X');
   EXPECT_EQ("R", freed);
   freed.Remove('R');
   EXPECT_EQ("", freed);
 
   CFX_ByteString empty;
   empty.Remove('X');
   EXPECT_EQ("", empty);
 }
 
+TEST(fxcrt, ByteStringRemoveCopies) {
+  CFX_ByteString freed("FREED");
+  const FX_CHAR* old_buffer = freed.c_str();
+
+  // No change with single reference - no copy.
+  freed.Remove('Q');
+  EXPECT_EQ("FREED", freed);
+  EXPECT_EQ(old_buffer, freed.c_str());
+
+  // Change with single reference - no copy.
+  freed.Remove('E');
+  EXPECT_EQ("FRD", freed);
+  EXPECT_EQ(old_buffer, freed.c_str());
+
+  // No change with multiple references - no copy.
+  CFX_ByteString shared(freed);
+  freed.Remove('Q');
+  EXPECT_EQ("FRD", freed);
+  EXPECT_EQ(old_buffer, freed.c_str());
+  EXPECT_EQ(old_buffer, shared.c_str());
+
+  // Change with multiple references -- must copy.
+  freed.Remove('D');
+  EXPECT_EQ("FR", freed);
+  EXPECT_NE(old_buffer, freed.c_str());
+  EXPECT_EQ("FRD", shared);
+  EXPECT_EQ(old_buffer, shared.c_str());
+}
+
 TEST(fxcrt, ByteStringReplace) {
   CFX_ByteString fred("FRED");
   fred.Replace("FR", "BL");
   EXPECT_EQ("BLED", fred);
   fred.Replace("D", "DDY");
   EXPECT_EQ("BLEDDY", fred);
   fred.Replace("LEDD", "");
   EXPECT_EQ("BY", fred);
   fred.Replace("X", "CLAMS");
   EXPECT_EQ("BY", fred);
@@ skipping 614 matching lines @@
   }
 }
 
 TEST(fxcrt, EmptyByteString) {
   CFX_ByteString empty_str;
   EXPECT_TRUE(empty_str.IsEmpty());
   EXPECT_EQ(0, empty_str.GetLength());
   const FX_CHAR* cstr = empty_str.c_str();
   EXPECT_EQ(0, FXSYS_strlen(cstr));
 }