Strings library

Characters

In the C++ standard library, a character is an object which, when treated sequentially, can represent text.

The term means not only objects of 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:

regular expressions library

(since C++11)

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.

Library components

The C++ strings library includes the following components:

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).

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)

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.

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).

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

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Null-terminated strings
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basic_string
basic_string_view (C++17)
char_traits