std::indirectly_writable(3) | C++ Standard Libary | std::indirectly_writable(3) |
NAME¶
std::indirectly_writable - std::indirectly_writable
Synopsis¶
Defined in header <iterator>
template<class Out, class T>
concept indirectly_writable =
requires(Out&& o, T&& t) {
*o = std::forward<T>(t);
*std::forward<Out>(o) = std::forward<T>(t); (since
const_cast<const std::iter_reference_t<Out>&&>(*o) =
std::forward<T>(t); C++20)
const_cast<const
std::iter_reference_t<Out>&&>(*std::forward<Out>(o))
=
std::forward<T>(t);
};
// none of the four expressions above are required to be
equality-preserving
The concept indirectly_writable<Out, T> specifies the requirements for
writing a
value whose type and value category are encoded by T into an iterator Out's
referenced object.
Semantic requirements
Let e be an expression such that decltype((e)) is T, and o be a
dereferenceable
object of type Out, then indirectly_writable<Out, T> is modeled only
if:
* If std::indirectly_readable<Out> is modeled and
std::iter_value_t<Out> is the
same type as std::decay_t<T>, then *o after any above assignment is
equal to the
value of e before the assignment.
o is not required to be dereferenceable after evaluating any of the
assignment
expressions above. If e is an xvalue, the resulting state of the object it
denotes
is valid but unspecified.
Equality preservation
An expression is equality preserving if it results in equal outputs given
equal
inputs.
* The inputs to an expression consist of its operands.
* The outputs of an expression consist of its result and all operands
modified by
the expression (if any).
In specification of standard concepts, operands are defined as the largest
subexpressions that include only:
* an id-expression, and
* invocations of std::move, std::forward, and std::declval.
The cv-qualification and value category of each operand is determined by
assuming
that each template type parameter denotes a cv-unqualified complete non-array
object
type.
Every expression required to be equality preserving is further required to be
stable: two evaluations of such an expression with the same input objects
must have
equal outputs absent any explicit intervening modification of those input
objects.
Notes¶
The only valid use of operator* is on the left side of an
assignment expression.
Assignment through the same value of an indirectly writable type may happen
only
once.
The required expressions with const_cast prevent indirectly_readable objects
with
prvalue reference types from satisfying the syntactic requirements of
indirectly_writable by accident, while permitting proxy references to
continue to
work as long as their constness is shallow. See Ranges TS issue 381.
2022.07.31 | http://cppreference.com |