BString: add support for move semantics with C++11 and up.

This implements the "rule of 5" for this type. While the copy operation for
BString was already using shallow copies of the underlying data, this change
further optimizes moving the data from one object to another.

While it is not the intention to implement move semantics to all types in the
legacy Haiku/Be kits, data types like BString are good candidates, because move
operations are often useful when working with data within an application.

In this implementation, the internal data of the string object will be set to
NULL, thus leaving an empty string.

Change-Id: I16bf9424f9b17f622b0b57659b80628e18760288
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4428
Reviewed-by: Jérôme Duval <[email protected]>

docs/user/support/String.dox

@@ -40,7 +40,7 @@
 	strings. For all operations that perform on bytes, there is an equivalent
 	that operates on UTF-8 strings. See for example the BString::CopyInto() and
 	BString::CopyCharsInto() methods. The main difference is that if there are
-	any position argumens, the regular method counts the bytes and the 
+	any position argumens, the regular method counts the bytes and the
 	Chars methods counts characters.
 
 	\since BeOS R5
@@ -94,6 +94,21 @@
 */
 
 
+#if __cplusplus >= 201103L
+/*!
+	\fn BString::BString(BString&& string)
+	\brief Move the data from the \a string to this object.
+
+	Create a new string object with the data of another \a string. The
+	\a string will no longer point to the same contents.
+
+	\note This constructor is only available for modern C++ (C++11 or later).
+
+	\since Haiku R1
+*/
+#endif
+
+
 /*!
 	\fn BString::~BString()
 	\brief Free all resources associated with the object.
@@ -270,6 +285,22 @@
 */
 
 
+#if __cplusplus >= 201103L
+/*!
+	\fn BString& BString::operator=(BString&& string)
+	\brief Move the contents of \a string to this BString object.
+
+	The \a string will no longer point to the same contents.
+
+	\note This method is only available for modern C++ (C++11 or later).
+
+	\return This method always returns \c *this.
+
+	\since Haiku R1
+*/
+#endif
+
+
 /*!
 	\fn BString& BString::SetTo(const char* str)
 	\brief Re-initialize the BString to a copy of the data of a string.