NAME¶

std::assignable_from - std::assignable_from

Synopsis¶

Defined in header <concepts>
template< class LHS, class RHS >

concept assignable_from =
std::is_lvalue_reference_v<LHS> &&
std::common_reference_with<
const std::remove_reference_t<LHS>&, (since C++20)
const std::remove_reference_t<RHS>&> &&
requires(LHS lhs, RHS&& rhs) {
{ lhs = std::forward<RHS>(rhs) } -> std::same_as<LHS>;

};

The concept assignable_from<LHS, RHS> specifies that an expression of the type and
value category specified by RHS can be assigned to an lvalue expression whose type
is specified by LHS.

Semantic requirements

Given

* lhs, an lvalue that refers to an object lcopy such that decltype((lhs)) is LHS,
* rhs, an expression such that decltype((rhs)) is RHS,
* rcopy, a distinct object that is equal to rhs,

assignable_from<LHS, RHS> is modeled only if

* std::addressof(lhs = rhs) == std::addressof(lcopy) (i.e., the assignment
expression yields an lvalue referring to the left operand);
* After evaluating lhs = rhs:

* lhs is equal to rcopy, unless rhs is a non-const xvalue that refers to
lcopy (i.e., the assignment is a self-move-assignment),
* if rhs is a glvalue:

* If it is a non-const xvalue, the object to which it refers is in a
valid but unspecified state;
* Otherwise, the object it refers to is not modified;

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.

Unless noted otherwise, every expression used in a requires-expression is required
to be equality preserving and stable, and the evaluation of the expression may
modify only its non-constant operands. Operands that are constant must not be
modified.

Notes¶

Assignment need not be a total function. In particular, if assigning to some object
x can cause some other object y to be modified, then x = y is likely not in the
domain of =. This typically happens if the right operand is owned directly or
indirectly by the left operand (e.g., with smart pointers to nodes in an node-based
data structure, or with something like std::vector<std::any>).

See also¶

is_assignable
is_trivially_assignable checks if a type has a assignment operator for a specific
is_nothrow_assignable argument
(C++11) (class template)
(C++11)
(C++11)