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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 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)

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