NAME

std::unique_ptr::operator= - std::unique_ptr::operator=

Synopsis

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 > (2) (constexpr since C++23)
unique_ptr& operator=( unique_ptr<U,E>&& r ) noexcept;
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 > (2) (constexpr since C++23)
unique_ptr& operator=( unique_ptr<U,E>&& r ) noexcept;
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.

The move assignment operator only participates in overload resolution if
std::is_move_assignable<Deleter>::value is true.
2) Converting assignment operator. Behaves same as (1), except that

* This assignment operator of the primary template only participates in overload
resolution if U is not an array type and unique_ptr<U,E>::pointer is implicitly
convertible to pointer and 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:

* U is an array type
* pointer is the same type as element_type*
* unique_ptr<U,E>::pointer is the same type as
unique_ptr<U,E>::element_type*
* unique_ptr<U,E>::element_type(*)[] is convertible to element_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 accepts
qualification conversions
LWG 2228 C++11 the converting assignment operator was not constrained
constrained
LWG 2899 C++11 the move assignment operator was not constrained
constrained