Merge string_ref to release

[SVN r82047]

doc/string_ref.qbk

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+[/
+ / Copyright (c) 2012 Marshall Clow
+ /
+ / Distributed under the Boost Software License, Version 1.0. (See accompanying
+ / file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+ /]
+
+[article String_Ref
+    [quickbook 1.5]
+    [authors [Clow, Marshall]]
+    [copyright 2012 Marshall Clow]
+    [license
+        Distributed under the Boost Software License, Version 1.0.
+        (See accompanying file LICENSE_1_0.txt or copy at
+        [@http://www.boost.org/LICENSE_1_0.txt])
+    ]
+]
+
+[/===============]
+[section Overview]
+[/===============]
+
+Boost.StringRef is an implementation of Jeffrey Yaskin's [@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3442.html N3442:
+string_ref: a non-owning reference to a string]. 
+
+When you are parsing/processing strings from some external source, frequently you want to pass a piece of text to a procedure for specialized processing. The canonical way to do this is as a `std::string`, but that has certain drawbacks:
+
+1) If you are processing a buffer of text (say a HTTP response or the contents of a file), then you have to create the string from the text you want to pass, which involves memory allocation and copying of data.
+
+2) if a routine receives a constant `std::string` and wants to pass a portion of that string to another routine, then it must create a new string of that substring.
+
+3) A routine receives a constant `std::string` and wants to return a portion of the string, then it must create a new string to return.
+
+`string_ref` is designed to solve these efficiency problems. A `string_ref` is a read-only reference to a contiguous sequence of characters, and provides much of the functionality of `std::string`. A `string_ref` is cheap to create, copy and pass by value, because it does not actually own the storage that it points to. 
+
+A `string_ref` is implemented as a small struct that contains a pointer to the start of the character data and a count. A `string_ref` is cheap to create and cheap to copy.
+
+`string_ref` acts as a container; it includes all the methods that you would expect in a container, including iteration support, `operator []`, `at` and `size`. It can be used with any of the iterator-based algorithms in the STL - as long as you don't need to change the underlying data (`sort` and `remove`, for example, will not work)
+
+Besides generic container functionality, `string_ref` provides a subset of the interface of `std::string`. This makes it easy to replace parameters of type `const std::string &` with `boost::string_ref`. 
+
+Because a `string_ref` does not own the data that it "points to", it introduces lifetime issues into code that uses it. The programmer must ensure that the data that a `string_ref` refers to exists as long as the `string_ref` does.
+
+[endsect]
+
+
+[/===============]
+[section Examples]
+[/===============]
+
+Integrating `string_ref` into your code is fairly simple. Wherever you pass a `const std::string &` or `std::string` as a parameter, that's a candidate for passing a `boost::string_ref`.
+
+	std::string extract_part ( const std::string &bar ) {
+		return bar.substr ( 2, 3 );
+		}
+		
+	if ( extract_part ( "ABCDEFG" ).front() == "C" ) { /* do something */ }
+	
+Let's figure out what happens in this (contrived) example.
+
+First, a temporary string is created from the string literal `"ABCDEFG"`, and it is passed (by reference) to the routine `extract_part`. Then a second string is created in the call `std::string::substr` and returned to `extract_part` (this copy may be elided by RVO). Then `extract_part` returns that string back to the caller (again this copy may be elided). The first temporary string is deallocated, and `front` is called on the second string, and then it is deallocated as well.
+
+Two `std::string`s are created, and two copy operations. That's (potentially) four memory allocations and deallocations, and the associated copying of data.
+
+Now let's look at the same code with `string_ref`:
+
+	boost::string_ref extract_part ( boost::string_ref bar ) {
+		return bar.substr ( 2, 3 );
+		}
+		
+	if ( extract_part ( "ABCDEFG" ).front() == "C" ) { /* do something */ }
+
+No memory allocations. No copying of character data. No changes to the code other than the types. There are two `string_ref`s created, and two `string_ref`s copied, but those are cheap operations.
+
+[endsect]
+
+
+[/=================]
+[section:reference Reference ]
+[/=================]
+
+The header file "string_ref.hpp" defines a template `boost::basic_string_ref`, and four specializations - for `char` / `wchar_t` / `char16_t` / `char32_t` .
+
+`#include <boost/utility/string_ref.hpp>`
+
+[endsect]
+
+[/===============]
+[section History]
+[/===============]
+
+[heading boost 1.53]
+* Introduced
+ 	
+
+[endsect]
+
+
+
+