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std::assignable_from(3) C++ Standard Libary 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 expres- sion whose type is specified by LHS. Semantic requirements Given * lhs, an lvalue that refers to an object lcopy such that de- cltype((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 deter- mined 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 re- quired 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-expres- sion is required to be equality preserving and stable, and the evaluation of the ex- pression 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) http://cppreference.com 2022.07.31 std::assignable_from(3)
NAME | Synopsis | Notes | See also
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