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233 lines
12 KiB
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233 lines
12 KiB
Plaintext
[/
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/ Copyright (c) 2012 Marshall Clow
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/ Copyright (c) 2015 Beman Dawes
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/
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/ Distributed under the Boost Software License, Version 1.0. (See accompanying
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/ file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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/]
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[article String_View
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[quickbook 1.5]
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[authors [Clow, Marshall] [Dawes, Beman]]
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[copyright 2012 Marshall Clow, 2015 Beman Dawes]
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[license
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Distributed under the Boost Software License, Version 1.0.
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[@http://www.boost.org/LICENSE_1_0.txt]
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]
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]
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[/===============]
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[section Overview]
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[/===============]
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Boost.StringView is an implementation of `string_view` as specified in [@
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http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4480.html#string.view N4480:
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ISO/IEC DTS 19568, Technical Specification - C++ Extensions for Library Fundamentals].
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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:
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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.
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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.
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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.
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`string_view` is designed to solve these efficiency problems. A `string_view` is a read-only reference to a contiguous sequence of characters, and provides much of the functionality of `std::string`. A `string_view` is cheap to create, copy and pass by value, because it does not actually own the storage that it points to.
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A `string_view` is implemented as a small struct that contains a pointer to the start of the character data and a count. A `string_view` is cheap to create and cheap to copy.
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`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 don't need to change the underlying data (`sort` and `remove`, for example, will not work)
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Besides generic container functionality, `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`, `string_view` has a static member variable named `npos` to denote the result of failed searches, and to mean "the end".
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Because a `string_view` 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_view` refers to exists as long as the `string_view` does.
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Note: The header actually contains a class template, `basic_string_view` and four typedefs:
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template<class charT, class traits = char_traits<charT>>
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class basic_string_view;
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typedef basic_string_view<char> string_view;
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typedef basic_string_view<char16_t> u16string_view;
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typedef basic_string_view<char32_t> u32string_view;
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typedef basic_string_view<wchar_t> wstring_view;
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So you can have views of strings of any of the four built-in character types as well as strings of user-defined character-like type strings. For the sake of simple exposition, we concentrate on `string_view` (i.e. `char` strings) in this documentation.
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[endsect]
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[/===============]
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[section Examples]
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[/===============]
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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`.
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std::string extract_part ( const std::string &bar ) {
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return bar.substr ( 2, 3 );
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}
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if ( extract_part ( "ABCDEFG" ).front() == 'C' ) { /* do something */ }
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Let's figure out what happens in this (contrived) example.
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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.
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Two `std::string`s are created, and two copy operations. That's (potentially) four memory allocations and deallocations, and the associated copying of data.
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Now let's look at the same code with `string_view`:
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boost::string_view extract_part ( boost::string_view bar ) {
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return bar.substr ( 2, 3 );
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}
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if ( extract_part ( "ABCDEFG" ).front() == "C" ) { /* do something */ }
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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.
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[endsect]
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[/=================]
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[section:reference Reference ]
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[/=================]
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The header file "string_view.hpp" defines a class template `boost::basic_string_view`, and four specializations - for `char` / `wchar_t` / `char16_t` / `char32_t` .
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`#include <boost/utility/string_view.hpp>`
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Types:
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typedef traits traits_type;
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typedef charT value_type;
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typedef charT* pointer;
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typedef const charT* const_pointer;
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typedef charT& reference;
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typedef const charT& const_reference;
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typedef const_pointer const_iterator; // implementation defined
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typedef const_iterator iterator;
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typedef std::reverse_iterator<const_iterator> const_reverse_iterator;
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typedef const_reverse_iterator reverse_iterator;
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typedef std::size_t size_type;
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typedef std::ptrdiff_t difference_type;
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static BOOST_CONSTEXPR_OR_CONST size_type npos = size_type(-1);
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Construction and copying:
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BOOST_CONSTEXPR basic_string_view () BOOST_NOEXCEPT; // Constructs empty string_view
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BOOST_CONSTEXPR basic_string_view (const basic_string_view &rhs) BOOST_NOEXCEPT;
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basic_string_view& operator=(const basic_string_view &rhs) BOOST_NOEXCEPT;
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template<typename Allocator>
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basic_string_view(const std::basic_string<charT, traits, Allocator>& str) BOOST_NOEXCEPT; // Ctor from std::string
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BOOST_CONSTEXPR basic_string_view(const charT* str); // Ctor from NULL-terminated string
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BOOST_CONSTEXPR basic_string_view(const charT* str, size_type len); // Ctor from pointer, length pair
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`string_view` does not define a move constructor or a move-assignment operator because copying a `string_view` is just as cheap as moving one would be.
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Basic container-like functions:
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BOOST_CONSTEXPR size_type size() const BOOST_NOEXCEPT ;
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BOOST_CONSTEXPR size_type length() const BOOST_NOEXCEPT ;
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BOOST_CONSTEXPR size_type max_size() const BOOST_NOEXCEPT ;
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BOOST_CONSTEXPR bool empty() const BOOST_NOEXCEPT ;
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// All iterators are const_iterators
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BOOST_CONSTEXPR const_iterator begin() const BOOST_NOEXCEPT ;
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BOOST_CONSTEXPR const_iterator cbegin() const BOOST_NOEXCEPT ;
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BOOST_CONSTEXPR const_iterator end() const BOOST_NOEXCEPT ;
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BOOST_CONSTEXPR const_iterator cend() const BOOST_NOEXCEPT ;
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const_reverse_iterator rbegin() const BOOST_NOEXCEPT ;
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const_reverse_iterator crbegin() const BOOST_NOEXCEPT ;
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const_reverse_iterator rend() const BOOST_NOEXCEPT ;
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const_reverse_iterator crend() const BOOST_NOEXCEPT ;
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Access to the individual elements (all of which are const):
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BOOST_CONSTEXPR const charT& operator[](size_type pos) const ;
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BOOST_CONSTEXPR const charT& at(size_t pos) const ;
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BOOST_CONSTEXPR const charT& front() const ;
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BOOST_CONSTEXPR const charT& back() const ;
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BOOST_CONSTEXPR const charT* data() const BOOST_NOEXCEPT ;
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Modifying the `string_view` (but not the underlying data):
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BOOST_CONSTEXPR void clear(); // boost extension
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BOOST_CONSTEXPR void remove_prefix(size_type n);
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BOOST_CONSTEXPR void remove_suffix(size_type n);
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BOOST_CONSTEXPR void swap(basic_string_view& s) BOOST_NOEXCEPT;
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Searching:
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BOOST_CONSTEXPR size_type find(basic_string_view s, size_type pos = 0) const BOOST_NOEXCEPT;
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BOOST_CONSTEXPR size_type find(charT c, size_type pos = 0) const BOOST_NOEXCEPT;
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BOOST_CONSTEXPR size_type find(const charT* s, size_type pos, size_type n) const;
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BOOST_CONSTEXPR size_type find(const charT* s, size_type pos = 0) const;
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BOOST_CONSTEXPR size_type rfind(basic_string_view s, size_type pos = npos) const BOOST_NOEXCEPT;
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BOOST_CONSTEXPR size_type rfind(charT c, size_type pos = npos) const BOOST_NOEXCEPT;
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BOOST_CONSTEXPR size_type rfind(const charT* s, size_type pos, size_type n) const;
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BOOST_CONSTEXPR size_type rfind(const charT* s, size_type pos = npos) const;
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BOOST_CONSTEXPR size_type find_first_of(basic_string_view s, size_type pos = 0) const BOOST_NOEXCEPT;
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BOOST_CONSTEXPR size_type find_first_of(charT c, size_type pos = 0) const BOOST_NOEXCEPT;
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BOOST_CONSTEXPR size_type find_first_of(const charT* s, size_type pos, size_type n) const;
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BOOST_CONSTEXPR size_type find_first_of(const charT* s, size_type pos = 0) const;
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BOOST_CONSTEXPR size_type find_last_of(basic_string_view s, size_type pos = npos) const BOOST_NOEXCEPT;
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BOOST_CONSTEXPR size_type find_last_of(charT c, size_type pos = npos) const BOOST_NOEXCEPT;
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BOOST_CONSTEXPR size_type find_last_of(const charT* s, size_type pos, size_type n) const;
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BOOST_CONSTEXPR size_type find_last_of(const charT* s, size_type pos = npos) const;
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BOOST_CONSTEXPR size_type find_first_not_of(basic_string_view s, size_type pos = 0) const BOOST_NOEXCEPT;
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BOOST_CONSTEXPR size_type find_first_not_of(charT c, size_type pos = 0) const BOOST_NOEXCEPT;
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BOOST_CONSTEXPR size_type find_first_not_of(const charT* s, size_type pos, size_type n) const;
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BOOST_CONSTEXPR size_type find_first_not_of(const charT* s, size_type pos = 0) const;
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BOOST_CONSTEXPR size_type find_last_not_of(basic_string_view s, size_type pos = npos) const BOOST_NOEXCEPT;
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BOOST_CONSTEXPR size_type find_last_not_of(charT c, size_type pos = npos) const BOOST_NOEXCEPT;
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BOOST_CONSTEXPR size_type find_last_not_of(const charT* s, size_type pos, size_type n) const;
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BOOST_CONSTEXPR size_type find_last_not_of(const charT* s, size_type pos = npos) const;
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BOOST_CONSTEXPR bool starts_with(charT c) const ; // boost extension
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BOOST_CONSTEXPR bool starts_with(basic_string_view x) const ; // boost extension
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BOOST_CONSTEXPR bool ends_with(charT c) const ; // boost extension
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BOOST_CONSTEXPR bool ends_with(basic_string_view x) const ; // boost extension
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String-like operations:
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template<class Allocator> // Only present if compiler supports C++11 explicit conversion
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explicit operator basic_string<charT, traits, Allocator>() const;
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template<class Allocator = allocator<charT> > // Default only for C++11 compilers
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basic_string<charT, traits, Allocator>
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to_string(const Allocator& a = Allocator()) const;
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size_type copy(charT* s, size_type n, size_type pos = 0) const;
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BOOST_CONSTEXPR basic_string_view substr(size_type pos, size_type n=npos) const ; // Creates new string_view
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Comparison:
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To be supplied
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[endsect]
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[/===============]
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[section History]
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[/===============]
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[heading boost 1.53]
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* Introduced, based on Jeffrey Yaskin's [@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3442.html N3442:
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string_ref: a non-owning reference to a string]
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[heading boost 1.60]
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* Updated to reflect [@
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http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4480.html#string.view N4480:
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ISO/IEC DTS 19568, Technical Specification - C++ Extensions for Library Fundamentals]
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[endsect]
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