C++ named requirements: CharTraits

C++

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Named requirements

Language support library

Concepts library (C++20)

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Regular expressions library (C++11)

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Technical Specifications

C++ named requirements

Basic
DefaultConstructible CopyAssignable CopyConstructible	MoveAssignable (C++11) MoveConstructible (C++11) Destructible
Layout
TrivialType (C++11) TriviallyCopyable (C++11)	StandardLayoutType (C++11) PODType
Library-Wide
EqualityComparable LessThanComparable Swappable (C++11) ValueSwappable (C++11) NullablePointer (C++11) Hash (C++11)	Allocator FunctionObject Callable Predicate BinaryPredicate Compare
Container
Container SequenceContainer ContiguousContainer (C++17) ReversibleContainer	AssociativeContainer AllocatorAwareContainer UnorderedAssociativeContainer (C++11)
Container Elements
DefaultInsertable (C++11) CopyInsertable (C++11) MoveInsertable (C++11)	EmplaceConstructible (C++11) Erasable (C++11)
Iterator
Iterator OutputIterator InputIterator ForwardIterator	BidirectionalIterator RandomAccessIterator ContiguousIterator (C++17) ConstexprIterator (C++20)
Stream I/O
FormattedInputFunction UnformattedInputFunction	FormattedOutputFunction UnformattedOutputFunction
Random Numbers
SeedSequence (C++11) RandomNumberEngine (C++11)	RandomNumberDistribution (C++11) UniformRandomBitGenerator (C++11) RandomNumberEngineAdaptor (C++11)
Concurrency
BasicLockable (C++11) Lockable (C++11) TimedLockable (C++11)	Mutex (C++11) TimedMutex (C++11) SharedMutex (C++17) SharedTimedMutex (C++14)
Other
CharTraits RegexTraits (C++11) BitmaskType NumericType LiteralType (C++11)	UnaryTypeTrait (C++11) BinaryTypeTrait (C++11) TransformationTrait (C++11) Clock (C++11) TrivialClock (C++11)

CharTraits is a traits class that abstracts basic character and string operations for a given character type. Most standard library string and input/output classes require a CharTraits template type parameter alongside a corresponding character template type parameter.

Requirements

No operation on CharTraits may throw an exception.

Given

• CharT, a character type
• X, a CharTraits type for type CharT
• c, d, values of type CharT
• p, q, values of type const CharT*
• s, a value of type CharT*
• n, i, j, values of type std::size_t
• e, f, values of type X::int_type
• pos, a value of type X::pos_type
• state, a value of type X::state_type
• r, an lvalue of type CharT

expression	return type	semantics	complexity
X::char_type	CharT	Used to refer to the character type	Compile-time
X::int_type		An type that can hold all valid values of X::char_type plus X::eof()	Compile-time
X::off_type		Invokes implementation-defined behaviour if not std::streamoff when X is used as the traits template parameter in input/output classes.	Compile-time
X::pos_type		Functions in input/output classes returning this type use X::pos_type(X::off_type(-1)) is an invalid value to signal an error Use of this invalid value as an argument to any std::istream, std::ostream or std::streambuf member taking a value of this type is undefined behaviour Invokes implementation-defined behaviour if this type is not std::streampos when X is used as the traits template parameter in input/output classes	Compile-time
X::state_type		CopyAssignable, CopyConstructible, DefaultConstructible	Compile-time
X::eq(c,d)	bool	Returns: whether c is to be treated as equal to d	Constant
X::lt(c,d)	bool	Returns: whether c is to be treated as less than d	Constant
X::compare(p,q,n)	int	Returns: 0 if for each i in [0,n), X::eq(p[i], q[i]) is true Else, a negative value if For some j in [0,n), X::lt(p[j], q[j]) is true and For each i in [0,j), X::eq(p[i], q[i]) is true Else a positive value	Linear
X::length(p)	std::size_t	Returns: the smallest i such that X::eq(p[i], CharT()) is true	Linear
X::find(p,n,c)	const X::char_type*	Returns: The smallest q in [p,p+n) such that X::eq(*q, c) is true Zero otherwise	Linear
X::move(s,p,n)	X::char_type*	For each i in [0,n), performs X::assign(s[i], p[i]) Copies correctly even where the ranges [p,p+n) and [s,s+n) overlap Returns: s	Linear
X::copy(s,p,n)	X::char_type*	Requires: p not in [s,s+n) Returns: s For each i in [0,n), performs X::assign(s[i], p[i])	Linear
X::assign(r,d)	(Not used)	Assigns r = d	Constant
X::assign(s,n,c)	X::char_type*	For each i in [0,n), performs X::assign(s[i], c). Returns: s	Linear
X::not_eof(e)	X::int_type	Returns: e if X::eq_int_type(e, X::eof()) is false Otherwise a value f such that X::eq_int_type(f, X::eof()) is false	Constant
X::to_char_type(e)	X::char_type	Returns: If for some c, X::eq_int_type(e, X::to_int_type(c)) is true, c Else some unspecified value	Constant
X::to_int_type(c)	X::int_type	Returns: some value e, constrained by the definitions of X::to_char_type and X::eq_int_type	Constant
X::eq_int_type(e,f)	bool	For all c and d, X::eq(c,d) is equal to X::eq_int_type(X::to_int_type(c), X::to_int_type(d)) Returns: Yields X::eq(c,d) if for some c and d, e == X::to_int_type(c) and f == X::to_int_type(d) Otherwise, yields true if e and f are both copies of X::eof() Otherwise, yields false if one of e and f is a copy of X::eof() and the other is not Otherwise the value is unspecified	Constant
X::eof()	X::int_type	Returns: a value e such that X::eq_int_type(e, X::to_int_type(c)) is false for all values c	Constant

Standard library

CharTraits is required by the following standard library class templates as a template type parameter:

basic_string	stores and manipulates sequences of characters (class template)
basic_string_view (C++17)	read-only string view (class template)

CharTraits is satisfied by the following standard library classes:

template<> class char_traits<char>; template<> class char_traits<wchar_t>; template<> class char_traits<char16_t>; template<> class char_traits<char32_t>;		(since C++11) (since C++11)