std::unique_ptr<T,Deleter>::operator=
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< cpp | memory | unique ptr
unique_ptr& operator=( unique_ptr&& r )noexcept; | (1) | (constexpr since C++23) |
template<class U, class E > unique_ptr& operator=( unique_ptr<U, E>&& r )noexcept; | (2) | (constexpr since C++23) |
unique_ptr& operator=(std::nullptr_t)noexcept; | (3) | (constexpr since C++23) |
unique_ptr& operator=(const unique_ptr&)= delete; | (4) | |
1) Move assignment operator. Transfers ownership from r to *this as if by calling reset(r.release()) followed by assigning get_deleter() from std::forward<Deleter>(r.get_deleter()).
This overload participates in overload resolution only if std::is_move_assignable<Deleter>::value is true.
If
Deleter is not a reference type, the behavior is undefined if Deleteris not MoveAssignable, or- assigning get_deleter() from an rvalue of type
Deleterwould throw an exception.
Otherwise (
Deleter is a reference type), the behavior is undefined if std::remove_reference<Deleter>::typeis not CopyAssignable, or- assigning get_deleter() from an lvalue of type
Deleterwould throw an exception.
2) Converting assignment operator. Transfers ownership from r to *this as if by calling reset(r.release()) followed by assigning get_deleter() from std::forward<E>(r.get_deleter()).
This overload participates in overload resolution only if all following conditions are satisfied:
- std::is_assignable<Deleter&, E&&>::value is true.
- For the primary template, all following conditions are satisfied:
Uis not an array type.unique_ptr<U, E>::pointeris implicitly convertible topointer, and.
- For the array specialization (
unique_ptr<T[]>), all following conditions are satisfied:Uis an array type.pointeris the same type aselement_type*.unique_ptr<U, E>::pointeris the same type asunique_ptr<U, E>::element_type*.unique_ptr<U, E>::element_type(*)[]is convertible toelement_type(*)[].
If
E is not a reference type, the behavior is undefined if assigning get_deleter() from an rvalue of type E is ill-formed or would throw an exception. Otherwise (
E is a reference type), the behavior is undefined if assigning get_deleter() from an lvalue of type E is ill-formed or would throw an exception.3) Effectively the same as calling reset().
4) Copy assignment operator is explicitly deleted.
Contents |
[edit]Parameters
| r | - | smart pointer from which ownership will be transferred |
[edit]Return value
*this
[edit]Notes
As a move-only type, unique_ptr's assignment operator only accepts rvalues arguments (e.g. the result of std::make_unique or a std::move'd unique_ptr variable).
[edit]Example
Run this code
#include <iostream>#include <memory> struct Foo {int id; Foo(int id): id(id){std::cout<<"Foo "<< id <<'\n';} ~Foo(){std::cout<<"~Foo "<< id <<'\n';}}; int main(){std::unique_ptr<Foo> p1(std::make_unique<Foo>(1)); {std::cout<<"Creating new Foo...\n";std::unique_ptr<Foo> p2(std::make_unique<Foo>(2));// p1 = p2; // Error ! can't copy unique_ptr p1 = std::move(p2);std::cout<<"About to leave inner block...\n"; // Foo instance will continue to live, // despite p2 going out of scope} std::cout<<"About to leave program...\n";}
Output:
Foo 1 Creating new Foo... Foo 2 ~Foo 1 About to leave inner block... About to leave program... ~Foo 2
[edit]Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 2047 | C++11 | for overload (2), get_deleter() was assigned from std::forward<Deleter>(r.get_deleter()) | corrected to std::forward<E>(r.get_deleter()) |
| LWG 2118 | C++11 | unique_ptr<T[]>::operator=rejected qualification conversions | accepts |
| LWG 2228 (N4366) | C++11 | the converting assignment operator was missing the assignability constraint | added the constraint |
| LWG 2246 | C++11 | the assignment target of the converted deleter of r was not specified | specified as get_deleter() |
| LWG 2899 | C++11 | the move assignment operator was not constrained | constrained |
