std::assignable_from(3)

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)