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C++ named requirements: CharTraits

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 listed below 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

Types

Type Semantics
X::char_type CharT
X::int_type A type that can hold all valid values of X::char_type plus X::eof()
X::off_type Invokes implementation-defined behavior if not std::streamoff when X is used as the traits template parameter in input/output classes.
X::pos_type
  • Functions in input/output classes returning this type use X::pos_type(X::off_type(-1)) as 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 behavior
  • Invokes implementation-defined behavior if this type is not std::streampos when X is used as the traits template parameter in input/output classes
X::state_type Destructible, CopyAssignable, CopyConstructible, DefaultConstructible

Expressions

Expression Return type Semantics Complexity
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
  • ​0​ 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, p + n) and [s, s + n) do not overlap
  • 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:

Strings
stores and manipulates sequences of characters
(class template)
(C++17)
read-only string view
(class template)
Streams
manages an arbitrary stream buffer
(class template)
wraps a given abstract device (std::basic_streambuf)
and provides high-level input interface
(class template)
implements high-level file stream input operations
(class template)
implements high-level string stream input operations
(class template)
(C++23)
implements fixed character buffer input operations
(class template)
wraps a given abstract device (std::basic_streambuf)
and provides high-level output interface
(class template)
implements high-level file stream output operations
(class template)
implements high-level string stream output operations
(class template)
(C++20)
synchronized output stream wrapper
(class template)
(C++23)
implements fixed character buffer output operations
(class template)
wraps a given abstract device (std::basic_streambuf)
and provides high-level input/output interface
(class template)
implements high-level file stream input/output operations
(class template)
implements high-level string stream input/output operations
(class template)
(C++23)
implements fixed character buffer input/output operations
(class template)
Stream iterators
input iterator that reads from std::basic_istream
(class template)
output iterator that writes to std::basic_ostream
(class template)
Stream buffers
abstracts a raw device
(class template)
implements raw file device
(class template)
implements raw string device
(class template)
(C++20)
synchronized output device wrapper
(class template)
(C++23)
implements raw fixed character buffer device
(class template)
Stream buffer iterators
input iterator that reads from std::basic_streambuf
(class template)
output iterator that writes to std::basic_streambuf
(class template)


CharTraits is satisfied by the following standard library explicit specializations of std::char_traits:

template<> class char_traits<char>;
template<> class char_traits<wchar_t>;
template<> class char_traits<char8_t>;
template<> class char_traits<char16_t>;
template<> class char_traits<char32_t>;

(since C++20)
(since C++11)
(since C++11)

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 335 C++98 the requirements on the binary overload of
assign did not prevent assignments to rvalues		 its first argument
can only be an lvalue
LWG 352 C++98 X::state_type was only
required to be CopyConstructible		 it is also required to be
CopyAssignable and DefaultConstructible
LWG 3085 C++98 X::copy(s, p, n) only required p not
in [s, s + n), which is too weak[1]		 requires [p, p + n) and
[s, s + n) not to overlap
  1. [p, p + n) and [s, s + n) can overlap, using std::memcpy to implement X::copy results in undefined behavior in this case.

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