std::assignable_from

 

C++

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Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

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Technical specifications

Symbols index

External libraries

 

Concepts library

Core language concepts same_as derived_from convertible_to common_reference_with      common_with integral signed_integral unsigned_integral floating_point swappableswappable_with destructible constructible_from default_initializable move_constructible copy_constructible assignable_from

Comparison concepts equality_comparableequality_comparable_with totally_orderedtotally_ordered_with Object concepts movable copyable semiregular regular Callable concepts invocableregular_invocable predicate relation equivalence_relation strict_weak_order

Exposition-only concepts boolean-testable

 

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

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

Expressions declared in requires-expressions of the standard library concepts are required to be equality-preserving (except where stated otherwise).

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 a node-based data structure, or with something like std::vector<std::any>).

See also

is_assignableis_trivially_assignableis_nothrow_assignable (C++11)(C++11)(C++11)

checks if a type has an assignment operator for a specific argument (class template)