std::assignable_from (since C++20)

 

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Technical specifications

Symbols index

External libraries

 

Concepts library

Core language concepts same_as (C++20) derived_from (C++20) convertible_to (C++20) common_reference_with (C++20) common_with (C++20) integral (C++20) signed_integral (C++20) unsigned_integral (C++20) floating_point (C++20) swappableswappable_with (C++20)(C++20) destructible (C++20) constructible_from (C++20) default_initializable (C++20)   move_constructible (C++20) copy_constructible (C++20) assignable_from (C++20)

Comparison concepts equality_comparableequality_comparable_with (C++20)(C++20) totally_orderedtotally_ordered_with (C++20)(C++20) Object concepts movable (C++20) copyable (C++20) semiregular (C++20) regular (C++20) Callable concepts invocableregular_invocable (C++20)(C++20) predicate (C++20) relation (C++20) equivalence_relation (C++20) strict_weak_order (C++20)

Exposition-only concepts boolean-testable  (C++20)

 

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>).

Example

Run this code

#include <atomic>
#include <concepts>
#include <string>
 
int main()
{
    // Normal basic usage, checks lvalue reference assignment
    static_assert(std::is_assignable_v<int&, int>);
    static_assert(std::assignable_from<int&, int>);
 
    static_assert(std::is_assignable_v<std::string&, std::string>);
    static_assert(std::assignable_from<std::string&, std::string>);
 
    // Fundamental types don't support assignment to an rvalue
    static_assert(!std::is_assignable_v<int, int>);
    static_assert(!std::assignable_from<int, int>);
 
    // std::assignable_from doesn't accept all valid assignment expressions:
 
    // rvalue reference assignment
    static_assert(std::is_assignable_v<std::string&&, std::string>);
    static_assert(!std::assignable_from<std::string&&, std::string>);
 
    // rvalue assignment
    static_assert(std::is_assignable_v<std::string, std::string>);
    static_assert(!std::assignable_from<std::string, std::string>);
 
    // std::atomic::operator= returns by value
    static_assert(std::is_assignable_v<std::atomic<int>&, int>);
    static_assert(!std::assignable_from<std::atomic<int>&, int>);
}

References

• C++23 standard (ISO/IEC 14882:2024):

• 18.4.8 Concept assignable_from [concept.assignable]

• C++20 standard (ISO/IEC 14882:2020):

• 18.4.8 Concept assignable_from [concept.assignable]

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)