[/ / Copyright (c) 2012 Marshall Clow / Copyright (c) 2021, Alan Freitas / / 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 String View] [/===============] [section Introduction] The class __boost_string_view__ and other classes derived from __basic_string_view__ represent references to strings or substrings. 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. Before __std_string_view__, the canonical way to do this used to be 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) If 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. __boost_string_view__ is designed to solve these efficiency problems. A __boost_string_view__ is a read-only reference to a contiguous sequence of characters, and provides much of the functionality of __std_string__. A __boost_string_view__ is cheap to create, copy and pass by value, because it does not actually own the storage that it points to. A __boost_string_view__ is implemented as a small struct that contains a pointer to the start of the character `data` and a `count`. A __boost_string_view__ is cheap to create and cheap to copy. __boost_string_view__ 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 do not need to change the underlying data. For example, __std_sort__ and __std_remove__ will not work. Besides generic container functionality, __boost_string_view__ provides a subset of the interface of __std_string__. This makes it easy to replace parameters of type `const __std_string__ &` with __boost_string_view__. Like __std_string__, __boost_string_view__ has a static member variable named `npos` to denote the result of failed searches, and to mean "the end". [caution Because a __boost_string_view__ does not own the data that it refers to, it introduces lifetime issues into code that uses it. The programmer must ensure that the data that a __string_view__ refers to exists as long as the __string_view__ does.] [note Boost.Utility also includes the class __string_ref__: - __string_ref__ is the initial 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]. - __string_view__ is an updated implementation to reflect the Library Fundamentals TS [@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4480.html N4480: \[string.view\]]. Please prefer __string_view__ / __basic_string_view__ over __string_ref__ / __basic_string_ref__: - The __basic_string_view__ class better matches __std_basic_string_view__. - __basic_string_view__ has WAY more constexpr support. - Code that uses __basic_string_ref__ should continue to work. - Not much code depends on __basic_string_ref__ anymore. ] [endsect] [/===============] [section Examples] [/===============] Integrating __string_view__ 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_view__. ``` __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 is potentially four memory allocations and deallocations, and the associated copying of data. Now let's look at the same code with __string_view__: ``` __boost_string_view__ extract_part ( __boost_string_view__ 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_view__ s created, and two __string_view__ s copied, but those are cheap operations. [endsect] [/=================] [section:reference Synopsis] [/=================] The header file [@../../../../boost/utility/string_view.hpp ``] defines a template __boost_basic_string_view__, and four specializations __string_view__, __wstring_view__, __u16string_view__, __u32string_view__ - for `char` / `wchar_t` / `char16_t` / `char32_t`. `#include ` Construction and copying: ``` constexpr basic_string_view (); // Constructs an empty string_view constexpr basic_string_view(const charT* str); // Constructs from a NULL-terminated string constexpr basic_string_view(const charT* str, size_type len); // Constructs from a pointer, length pair template basic_string_view(const __std_basic_string__& str); // Constructs from a std::string basic_string_view (const basic_string_view &rhs); basic_string_view& operator=(const basic_string_view &rhs); ``` __string_view__ does not define a move constructor nor a move-assignment operator because copying a __string_view__ is just a cheap as moving one. Basic container-like functions: ``` constexpr size_type size() const ; constexpr size_type length() const ; constexpr size_type max_size() const ; constexpr bool empty() const ; // All iterators are const_iterators constexpr const_iterator begin() const ; constexpr const_iterator cbegin() const ; constexpr const_iterator end() const ; 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): ``` constexpr const charT& operator[](size_type pos) const ; const charT& at(size_t pos) const ; constexpr const charT& front() const ; constexpr const charT& back() const ; constexpr const charT* data() const ; ``` Modifying the __string_view__ (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_view s) const ; size_type find(charT c) const ; size_type rfind(basic_string_view 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_view s) const ; size_type find_last_of(basic_string_view s) const ; size_type find_first_not_of(basic_string_view s) const ; size_type find_first_not_of(charT c) const ; size_type find_last_not_of(basic_string_view s) const ; size_type find_last_not_of(charT c) const ; ``` String-like operations: ``` constexpr basic_string_view substr(size_type pos, size_type n=npos) const ; // Creates a new string_view bool starts_with(charT c) const ; bool starts_with(basic_string_view x) const ; bool ends_with(charT c) const ; bool ends_with(basic_string_view x) const ; ``` [endsect] [/===============] [section History] [/===============] [h5 boost 1.71] * Glen Fernandes updated the implementation of the stream insertion operator to write directly to the `basic_streambuf` and refactored that functionality into a common utility. [h5 boost 1.53] * Introduced [endsect] [/===============] [xinclude tmp/string_view_reference.xml] [/===============] [/===============] [section Acknowledgments] [/===============] Author: Clow, Marshall Copyright 2012 Marshall Clow [endsect] [endsect]