std::unique_ptr<T,Deleter>::operator=
From cppreference.com
< cpp | memory | unique ptr
| members of the primary template, unique_ptr<T> |
||
| 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) |
| members of the specialization for arrays, unique_ptr<T[]> |
||
| 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) |
1) Move assignment operator. Transfers ownership from
r to *this as if by calling reset(r.release()) followed by an assignment of get_deleter() from std::forward<Deleter>(r.get_deleter()).
If Deleter is not a reference type, requires that it is nothrow-MoveAssignable.
If Deleter is a reference type, requires that std::remove_reference<Deleter>::type is nothrow-CopyAssignable.
2) Converting assignment operator. Behaves same as (1), except that
- This assignment operator of the primary template only participates in overload resolution if
Uis not an array type and unique_ptr<U,E>::pointer is implicitly convertible topointerand std::is_assignable<Deleter&, E&&>::value is true. - This assignment operator in the specialization for arrays,
std::unique_ptr<T[]>behaves the same as in the primary template, except that will only participate in overload resolution if all of the following is true:
-
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(*)[] - std::is_assignable<Deleter&, E&&>::value is true
-
3) Effectively the same as calling reset().
Note that unique_ptr's assignment operator only accepts rvalues, which are typically generated by std::move. (The unique_ptr class explicitly deletes its lvalue copy constructor and lvalue assignment operator.)
Parameters
| r | - | smart pointer from which ownership will be transferred |
Return value
*this
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
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 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 |