The conceptassignable_from<LHS, RHS> specifies that an expression of the type and value category specified byRHS can be assigned to an lvalue expression whose type is specified byLHS.

[edit]Semantic requirements

Given

• lhs, an lvalue that refers to an objectlcopy such thatdecltype((lhs)) isLHS,
• rhs, an expression such thatdecltype((rhs)) isRHS,
• rcopy, a distinct object that is equal torhs,

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 evaluatinglhs= rhs:
  • lhs is equal torcopy, unlessrhs is a non-const xvalue that refers tolcopy (i.e., the assignment is a self-move-assignment),
  • ifrhs 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;

[edit]Equality preservation

Expressions declared inrequires expressions of the standard library concepts are required to beequality-preserving (except where stated otherwise).

[edit]Notes

Assignment need not be a total function. In particular, if assigning to some objectx can cause some other objecty to be modified, thenx= 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 likestd::vector<std::any>).

[edit]Example

#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>);}

[edit]References

[edit]See also

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