| [/ |
| / 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`. 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. |
| |
| [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>` |
| |
| 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 ; |
| |
| [endsect] |
| |
| [/===============] |
| [section History] |
| [/===============] |
| |
| [heading boost 1.53] |
| * Introduced |
| |
| |
| [endsect] |
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