Bug 1431261 - Add nsTDependentString copy constructor. r=dbaron

This adds a copy constructor that uses Rebind to preserve the reference to
static data rather allocate a new shared buffer.

--HG--
extra : rebase_source : db04def1137a9be646b8e09dfee780c9163a585c

xpcom/string/nsTDependentString.h

@@ -105,11 +105,21 @@ public:
   {
   }
 
-  // XXX are you sure??
-  // auto-generated copy-constructor OK
-  // auto-generated copy-assignment operator OK
   // auto-generated destructor OK
 
+  nsTDependentString(self_type&& aStr)
+    : string_type()
+  {
+    Rebind(aStr, /* aStartPos = */ 0);
+    aStr.SetToEmptyBuffer();
+  }
+
+  explicit
+  nsTDependentString(const self_type& aStr)
+    : string_type()
+  {
+    Rebind(aStr, /* aStartPos = */ 0);
+  }
 
   /**
    * allow this class to be bound to a different string...
@@ -128,6 +138,7 @@ private:
 
   // NOT USED
   nsTDependentString(const substring_tuple_type&) = delete;
+  self_type& operator=(const self_type& aStr) = delete;
 };
 
 extern template class nsTDependentString<char>;

xpcom/tests/gtest/TestStrings.cpp

@@ -68,6 +68,52 @@ TEST(Strings, IsChar)
 #endif
 }
 
+TEST(Strings, DependentStrings)
+{
+  // A few tests that make sure copying nsTDependentStrings behaves properly.
+  using DataFlags = mozilla::detail::StringDataFlags;
+
+  {
+    // Test copy ctor.
+    nsDependentCString tmp("foo");
+    auto data = tmp.Data();
+    nsDependentCString foo(tmp);
+    // Neither string should be using a shared buffer.
+    EXPECT_FALSE(tmp.GetDataFlags() & DataFlags::SHARED);
+    EXPECT_FALSE(foo.GetDataFlags() & DataFlags::SHARED);
+    // Both strings should be pointing to the original buffer.
+    EXPECT_EQ(data, tmp.Data());
+    EXPECT_EQ(data, foo.Data());
+  }
+  {
+    // Test move ctor.
+    nsDependentCString tmp("foo");
+    auto data = tmp.Data();
+    nsDependentCString foo(mozilla::Move(tmp));
+    // Neither string should be using a shared buffer.
+    EXPECT_FALSE(tmp.GetDataFlags() & DataFlags::SHARED);
+    EXPECT_FALSE(foo.GetDataFlags() & DataFlags::SHARED);
+    // First string should be reset, the second should be pointing to the
+    // original buffer.
+    EXPECT_NE(data, tmp.Data());
+    EXPECT_EQ(data, foo.Data());
+    EXPECT_TRUE(tmp.IsEmpty());
+  }
+  {
+    // Test copying to a nsCString.
+    nsDependentCString tmp("foo");
+    auto data = tmp.Data();
+    nsCString foo(tmp);
+    // Original string should not be shared, copy should be shared.
+    EXPECT_FALSE(tmp.GetDataFlags() & DataFlags::SHARED);
+    EXPECT_TRUE(foo.GetDataFlags() & DataFlags::SHARED);
+    // First string should remain the same, the second should be pointing to
+    // a new buffer.
+    EXPECT_EQ(data, tmp.Data());
+    EXPECT_NE(data, foo.Data());
+  }
+}
+
 TEST(Strings, assign)
 {
   nsCString result;