std::unique_ptr<T,Deleter>::operator=
From cppreference.com
< 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()).
For the primary template, this overload participates in overload resolution only if
-
Uis not an array type, -
unique_ptr<U, E>::pointeris implicitly convertible topointer, and - std::is_assignable<Deleter&, E&&>::value is true.
For the array specialization (
unique_ptr<T[]>), this overload participates in overload resolution only if
-
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(*)[], and - std::is_assignable<Deleter&, E&&>::value is true.
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 | C++11 | the converting assignment operator was not constrained | constrained |
| LWG 2899 | C++11 | the move assignment operator was not constrained | constrained |
