18 Concepts library [concepts]

18.4 Language-related concepts [concepts.lang]

18.4.8 Concept assignable_­from [concept.assignable]

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template<class LHS, class RHS> concept assignable_­from = is_lvalue_reference_v<LHS> && common_reference_with<const remove_reference_t<LHS>&, const remove_reference_t<RHS>&> && requires(LHS lhs, RHS&& rhs) { { lhs = std::forward<RHS>(rhs) } -> same_­as<LHS>; };
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Let:
  • (1.1)
    lhs be an lvalue that refers to an object lcopy such that decltype((lhs)) is LHS,
  • (1.2)
    rhs be an expression such that decltype((rhs)) is RHS, and
  • (1.3)
    rcopy be a distinct object that is equal to rhs.
LHS and RHS model assignable_­from<LHS, RHS> only if
  • (1.4)
    addressof(lhs = rhs) == addressof(lcopy).
  • (1.5)
    After evaluating lhs = rhs:
    • (1.5.1)
      lhs is equal to rcopy, unless rhs is a non-const xvalue that refers to lcopy.
    • (1.5.2)
      If rhs is a non-const xvalue, the resulting state of the object to which it refers is valid but unspecified ([lib.types.movedfrom]).
    • (1.5.3)
      Otherwise, if rhs is a glvalue, the object to which it refers is not modified.
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[Note 1:
Assignment need not be a total function ([structure.requirements]); in particular, if assignment to an object x can result in a modification of some other object y, then x = y is likely not in the domain of =.
— end note]