Difference between revisions of "cpp/string"

Latest revision as of 09:45, 6 November 2024

In the strings library and regular expressions library(since C++11), a character can be of only char-like types, i.e. those non-array types that are also of PODType(until C++20)TrivialType and StandardLayoutType(since C++20) type. Therefore, characters are also referred as char-like objects in the strings library and regular expressions library(since C++11).

Some standard library components accept character container types. They, too, are types used to represent individual characters. Such types are used for one of the template arguments of std::char_traits and the class templates which use std::char_traits.

[edit] Library components

The C++ strings library includes the following components:

[edit] Character traits

Many character-related class templates (such as std::basic_string) need a set of related types and functions to complete the definition of their semantics. These types and functions are provided as a set of member typedef names and functions in the template parameter Traits used by each such template. The classes which are able to complete those semantics are CharTraits.

The string library provides the class template std::char_traits that defines types and functions for std::basic_string and std::basic_string_view(since C++17).

The following specializations are defined, all of them satisfy the CharTraits requirements:

Defined in header `<string>`
template<> class char_traits<char>;
template<> class char_traits<wchar_t>;
template<> class char_traits<char8_t>;		(since C++20)
template<> class char_traits<char16_t>;		(since C++11)
template<> class char_traits<char32_t>;		(since C++11)

When a user-defined character container type for std::basic_string and std::basic_string_view(since C++17) is used, it is also necessary to provide a corresponding character trait class (which can be a specialization of std::char_traits).

[edit] String classes (std::string etc.)

The class template std::basic_string generalizes how sequences of characters are manipulated and stored. String creation, manipulation, and destruction are all handled by a convenient set of class methods and related functions.

Several specializations of std::basic_string are provided for commonly-used types:

Defined in header `<string>`
Type	Definition
`std::string`	std::basic_string<char>
`std::wstring`	std::basic_string<wchar_t>
`std::u8string` (since C++20)	std::basic_string<char8_t>
`std::u16string` (since C++11)	std::basic_string<char16_t>
`std::u32string` (since C++11)	std::basic_string<char32_t>

String view classes (std::string_view etc.)

The class template std::basic_string_view provides a lightweight object that offers read-only access to a string or a part of a string using an interface similar to the interface of std::basic_string.

Several specializations of std::basic_string_view are provided for commonly-used types:

Defined in header `<string_view>`
Type	Definition
`std::string_view`	std::basic_string_view<char>
`std::wstring_view`	std::basic_string_view<wchar_t>
`std::u8string_view` (since C++20)	std::basic_string_view<char8_t>
`std::u16string_view`	std::basic_string_view<char16_t>
`std::u32string_view`	std::basic_string_view<char32_t>

(since C++17)

[edit] Null-terminated sequence utilities

Null-terminated character sequences (NTCTS) are sequences of characters that are terminated by a null character (the value after value-initialization).

The strings library provides functions to create, inspect, and modify such sequences:

null-terminated byte strings (NTBS) helper functions (including support of wide character types),
null-terminated multibyte strings (NTMBS) helper functions.

[edit] Relevant libraries

The localization library provides support for string conversions (e.g. std::toupper), character classification functions (e.g. std::isspace), and text encoding recognition (std::text_encoding).

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 1170	C++98	char-like types could be array types	prohibited

[edit] See also

C documentation for Strings library

@@ Line 2: / Line 2: @@
 {{cpp/string/navbar}}
-The C++ strings library includes support for three general types of strings:
+===Characters===
+In the C++ standard library, a ''character'' is an object which, when treated sequentially, can represent text.
-* {{lc|std::basic_string}} - a templated class designed to manipulate strings of any character type.
+The term means not only objects of [[cpp/language/type|character types]], but also any value that can be represented by a type that provides the definitions specified in the strings library and following libraries:
-* {{lc|std::basic_string_view}} {{mark c++17}} - a lightweight non-owning read-only view into a subsequence of a string.
+* [[cpp/locale|localization library]]
-* Null-terminated strings - arrays of characters terminated by a special ''null'' character.
+* [[cpp/io|input/output library]]
+{{rrev|since=c++11|
+* [[cpp/regex|regular expressions library]]
+}}
-=== {{lc|std::basic_string}} ===
+In the strings library{{rev inl|since=c++11| and regular expressions library}}, a character can be of only ''char-like types'', i.e. those non-array types that are also of {{rev inl|until=c++20|{{named req|PODType}}}}{{rev inl|since=c++20|{{named req|TrivialType}} and {{named req|StandardLayoutType}}}} type. Therefore, characters are also referred as ''char-like objects'' in the strings library{{rev inl|since=c++11| and regular expressions library}}.
-The templated class {{lc|std::basic_string}} generalizes how sequences of characters are manipulated and stored.  String creation, manipulation, and destruction are all handled by a convenient set of class methods and related functions.
+Some standard library components accept ''character container types''. They, too, are types used to represent individual characters. Such types are used for one of the template arguments of {{lc|std::char_traits}} and the class templates which use {{lc|std::char_traits}}.
+===Library components===
+The C++ strings library includes the following components:
+====Character traits====
+Many character-related class templates (such as {{lc|std::basic_string}}) need a set of related types and functions to complete the definition of their semantics. These types and functions are provided as a set of member {{c/core|typedef}} names and functions in the template parameter {{tt|Traits}} used by each such template. The classes which are able to complete those semantics are {{named req|CharTraits}}.
+The string library provides the class template {{lc|std::char_traits}} that defines types and functions for {{lc|std::basic_string}}{{rev inl|since=c++17| and {{lc|std::basic_string_view}}}}.
+The following specializations are defined, all of them satisfy the {{named req|CharTraits}} requirements:
+{{dcl begin}}
+{{dcl header|string}}
+{{dcl|template<> class char_traits<char>;}}
+{{dcl|template<> class char_traits<wchar_t>;}}
+{{dcl|template<> class char_traits<char8_t>;|since=c++20}}
+{{dcl|template<> class char_traits<char16_t>;|since=c++11}}
+{{dcl|template<> class char_traits<char32_t>;|since=c++11}}
+{{dcl end}}
+When a user-defined character container type for {{lc|std::basic_string}}{{rev inl|since=c++17| and {{lc|std::basic_string_view}}}} is used, it is also necessary to provide a corresponding character trait class (which can be a specialization of {{lc|std::char_traits}}).
+{{anchor|String classes}}
+====String classes ({{lc|std::string}} etc.)====
+The class template {{lc|std::basic_string}} generalizes how sequences of characters are manipulated and stored.  String creation, manipulation, and destruction are all handled by a convenient set of class methods and related functions.
 Several specializations of {{lc|std::basic_string}} are provided for commonly-used types:
 {{dsc begin}}
-{{dsc header | string}}
+{{dsc header|string}}
-{{dsc hitem | Type | Definition}}
+{{dsc hitem|Type|Definition}}
-{{dsc | {{lc|std::string}} | {{c|std::basic_string<char>}}}}
+{{dsc|{{ttb|std::string}}|{{c/core|std::basic_string<char>}}}}
-{{dsc | {{lc|std::wstring}} | {{c|std::basic_string<wchar_t>}}}}
+{{dsc|{{ttb|std::wstring}}|{{c/core|std::basic_string<wchar_t>}}}}
-{{dsc | {{lc|std::u8string}} {{mark since c++20}} | {{c|std::basic_string<char8_t>}}}}
+{{dsc|{{ttb|std::u8string}} {{mark since c++20}}|{{c/core|std::basic_string<char8_t>}}}}
-{{dsc | {{lc|std::u16string}} {{mark since c++11}} | {{c|std::basic_string<char16_t>}}}}
+{{dsc|{{ttb|std::u16string}} {{mark since c++11}}|{{c/core|std::basic_string<char16_t>}}}}
-{{dsc | {{lc|std::u32string}} {{mark since c++11}} | {{c|std::basic_string<char32_t>}}}}
+{{dsc|{{ttb|std::u32string}} {{mark since c++11}}|{{c/core|std::basic_string<char32_t>}}}}
 {{dsc end}}
 {{rrev|since=c++17|
-=== {{ltt|cpp/string/basic_string_view|std::basic_string_view}} ===
+{{anchor|String view classes}}
+====String view classes ({{lc|std::string_view}} etc.)====
-The templated class {{lc|std::basic_string_view}} provides a lightweight object that offers read-only access to a string or a part of a string using an interface similar to the interface of {{lc|std::basic_string}}.
+The class template {{lc|std::basic_string_view}} provides a lightweight object that offers read-only access to a string or a part of a string using an interface similar to the interface of {{lc|std::basic_string}}.
 Several specializations of {{lc|std::basic_string_view}} are provided for commonly-used types:
 {{dsc begin}}
-{{dsc header | string}}
+{{dsc header|string_view}}
-{{dsc hitem | Type | Definition}}
+{{dsc hitem|Type|Definition}}
-{{dsc | {{lc|std::string_view}} {{mark since c++17}} | {{c|std::basic_string_view<char>}}}}
+{{dsc|{{ttb|std::string_view}}|{{c/core|std::basic_string_view<char>}}}}
-{{dsc | {{lc|std::wstring_view}} {{mark since c++17}} | {{c|std::basic_string_view<wchar_t>}}}}
+{{dsc|{{ttb|std::wstring_view}}|{{c/core|std::basic_string_view<wchar_t>}}}}
-{{dsc | {{lc|std::u8string_view}} {{mark since c++20}} | {{c|std::basic_string_view<char8_t>}}}}
+{{dsc|{{ttb|std::u8string_view}} {{mark since c++20}}|{{c/core|std::basic_string_view<char8_t>}}}}
-{{dsc | {{lc|std::u16string_view}} {{mark since c++17}} | {{c|std::basic_string_view<char16_t>}}}}
+{{dsc|{{ttb|std::u16string_view}}|{{c/core|std::basic_string_view<char16_t>}}}}
-{{dsc | {{lc|std::u32string_view}} {{mark since c++17}} | {{c|std::basic_string_view<char32_t>}}}}
+{{dsc|{{ttb|std::u32string_view}}|{{c/core|std::basic_string_view<char32_t>}}}}
 {{dsc end}}
 }}
-=== Null-terminated strings ===
-Null-terminated strings are arrays of characters that are terminated by a special ''null'' character.  C++ provides functions to create, inspect, and modify null-terminated strings.
+====Null-terminated sequence utilities====
+''Null-terminated character sequences'' (NTCTS) are sequences of characters that are terminated by a null character (the value after [[cpp/language/value initialization|value-initialization]]).
-There are three types of null-terminated strings:
+The strings library provides functions to create, inspect, and modify such sequences:
+* {{rl|byte|null-terminated byte strings}} (NTBS) helper functions (including support of {{rl|wide|wide character types}}),
-* {{rl | byte | null-terminated byte strings}}
+* {{rl|multibyte|null-terminated multibyte strings}} (NTMBS) helper functions.
-* {{rl | multibyte | null-terminated multibyte strings}}
-* {{rl | wide | null-terminated wide strings}}
-=== Additional support ===
-===={{lc|std::char_traits}}====
-The string library also provides class template {{lc|std::char_traits}} that defines types and functions for {{lc|std::basic_string}} {{rev inl|since=c++17|and {{lc|std::basic_string_view}}}}. The following specializations are defined:
-{{dcl begin}}
-{{dcl header | string}}
-{{dcl | template<> class char_traits<char>; }}
-{{dcl | template<> class char_traits<wchar_t>; }}
-{{dcl | since=c++20 | template<> class char_traits<char8_t>; }}
-{{dcl | since=c++11 | template<> class char_traits<char16_t>; }}
-{{dcl | since=c++11 | template<> class char_traits<char32_t>; }}
-{{dcl end}}
-====Conversions and classification====
+===Relevant libraries===
+The [[cpp/locale|localization library]] provides support for string conversions (e.g. {{ltt std|cpp/locale/toupper}}), character classification functions (e.g. {{ltt std|cpp/locale/isspace}}), and text encoding recognition ({{ltt std|cpp/locale/text_encoding}}).
-The [[cpp/locale|localizations library]] provides support for string conversions (e.g. {{c|std::wstring_convert}} or {{c|std::toupper}}) as well as functions that classify characters (e.g. {{c|std::isspace}} or {{c|std::isdigit}}).
+===Defect reports===
+{{dr list begin}}
+{{dr list item|wg=lwg|dr=1170|std=C++98|before=char-like types could be array types|after=prohibited}}
+{{dr list end}}
 ===See also===
 {{dsc begin}}
-{{dsc see cpp | cpp/locale | Localizations library | nomono=true}}
+{{dsc see c|c/string|Strings library|nomono=true}}
-{{dsc see c | c/string | Strings library | nomono=true}}
 {{dsc end}}
 {{langlinks|ar|de|es|fr|it|ja|pt|ru|zh}}

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