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 typeX, a CharTraits type for type CharTc,d, values of type CharTp,q, values of type const CharT*s, a value of type CharT*n,i,j, values of type std::size_te,f, values of typeX::int_typepos, a value of typeX::pos_typestate, a value of typeX::state_typer, 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 | A type that can hold all valid values of X::char_type plus X::eof() | Compile-time | |
X::off_type | Invokes implementation-defined behavior 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)) 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 | Compile-time | |
X::state_type | Destructible, 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 * 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
| pub | basic_string | stores and manipulates sequences of characters |
| pub | basic_string_view(C++17) | read-only string view |
Streams
| pub | basic_ios | manages an arbitrary stream buffer |
| pub | basic_istream | wraps a given abstract device (std::basic_streambuf)and provides high-level input interface |
| pub | basic_ifstream | implements high-level file stream input operations |
| pub | basic_istringstream | implements high-level string stream input operations |
| pub | basic_ispanstream(C++23) | implements fixed character buffer input operations |
| pub | basic_ostream | wraps a given abstract device (std::basic_streambuf)and provides high-level output interface |
| pub | basic_ofstream | implements high-level file stream output operations |
| pub | basic_ostringstream | implements high-level string stream output operations |
| pub | basic_osyncstream(C++20) | synchronized output stream wrapper |
| pub | basic_ospanstream(C++23) | implements fixed character buffer output operations |
| pub | basic_iostream | wraps a given abstract device (std::basic_streambuf)and provides high-level input/output interface |
| pub | basic_fstream | implements high-level file stream input/output operations |
| pub | basic_stringstream | implements high-level string stream input/output operations |
| pub | basic_spanstream(C++23) | implements fixed character buffer input/output operations |
Stream iterators
| pub | istream_iterator | input iterator that reads from std::basic_istream |
| pub | ostream_iterator | output iterator that writes to std::basic_ostream |
Stream buffers
| pub | basic_streambuf | abstracts a raw device |
| pub | basic_filebuf | implements raw file device |
| pub | basic_stringbuf | implements raw string device |
| pub | basic_syncbuf(C++20) | synchronized output device wrapper |
| pub | basic_spanbuf(C++23) | implements raw fixed character buffer device |
Stream buffer iterators
| pub | istreambuf_iterator | input iterator that reads from std::basic_streambuf |
| pub | ostreambuf_iterator | output iterator that writes to std::basic_streambuf |
CharTraits is satisfied by the following standard library classes:
- 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)
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 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.