Boost C++ Libraries

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String_Ref

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)


Table of Contents

Overview
Examples
Reference
History

Boost.StringRef is an implementation of Jeffrey Yaskin's 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. Like std::string, string_ref has a static member variable named npos to denote the result of failed searches, and to mean "the end".

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.

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::strings 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_refs created, and two string_refs copied, but those are cheap operations.

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>

Construction and copying:

BOOST_CONSTEXPR basic_string_ref ();    // Constructs an empty string_ref
BOOST_CONSTEXPR basic_string_ref(const charT* str); // Constructs from a NULL-terminated string
BOOST_CONSTEXPR basic_string_ref(const charT* str, size_type len); // Constructs from a pointer, length pair
template<typename Allocator>
basic_string_ref(const std::basic_string<charT, traits, Allocator>& str); // Constructs from a std::string
basic_string_ref (const basic_string_ref &rhs);
basic_string_ref& operator=(const basic_string_ref &rhs);

string_ref does not define a move constructor nor a move-assignment operator because copying a string_ref is just a cheap as moving one.

Basic container-like functions:

BOOST_CONSTEXPR size_type size()     const ;
BOOST_CONSTEXPR size_type length()   const ;
BOOST_CONSTEXPR size_type max_size() const ;
BOOST_CONSTEXPR bool empty()         const ;

// All iterators are const_iterators
BOOST_CONSTEXPR const_iterator  begin() const ;
BOOST_CONSTEXPR const_iterator cbegin() const ;
BOOST_CONSTEXPR const_iterator    end() const ;
BOOST_CONSTEXPR const_iterator   cend() const ;
const_reverse_iterator         rbegin() const ;
const_reverse_iterator        crbegin() const ;
const_reverse_iterator           rend() const ;
const_reverse_iterator          crend() const ;

Access to the individual elements (all of which are const):

BOOST_CONSTEXPR const charT& operator[](size_type pos) const ;
const charT& at(size_t pos) const ;
BOOST_CONSTEXPR const charT& front() const ;
BOOST_CONSTEXPR const charT& back()  const ;
BOOST_CONSTEXPR const charT* data()  const ;

Modifying the string_ref (but not the underlying data):

void clear();
void remove_prefix(size_type n);
void remove_suffix(size_type n);

Searching:

size_type find(basic_string_ref s) const ;
size_type find(charT c) const ;
size_type rfind(basic_string_ref s) const ;
size_type rfind(charT c) const ;
size_type find_first_of(charT c) const ;
size_type find_last_of (charT c) const ;

size_type find_first_of(basic_string_ref s) const ;
size_type find_last_of(basic_string_ref s) const ;
size_type find_first_not_of(basic_string_ref s) const ;
size_type find_first_not_of(charT c) const ;
size_type find_last_not_of(basic_string_ref s) const ;
size_type find_last_not_of(charT c) const ;

String-like operations:

BOOST_CONSTEXPR basic_string_ref substr(size_type pos, size_type n=npos) const ; // Creates a new string_ref
bool starts_with(charT c) const ;
bool starts_with(basic_string_ref x) const ;
bool ends_with(charT c) const ;
bool ends_with(basic_string_ref x) const ;

boost 1.53

  • Introduced

Last revised: October 27, 2013 at 21:01:34 GMT