std::unique_ptr<T,Deleter>::operator=

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

Deleter is not MoveAssignable, or
assigning get_deleter() from an rvalue of type Deleter would throw an exception.

Otherwise (Deleter is a reference type), the behavior is undefined if

std::remove_reference<Deleter>::type is not CopyAssignable, or
assigning get_deleter() from an lvalue of type Deleter would 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

U is not an array type,
unique_ptr<U, E>::pointer is implicitly convertible to pointer, and
std::is_assignable<Deleter&, E&&>::value is true.

For the array specialization (unique_ptr<T[]>), this overload participates in overload resolution only if

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(*)[], 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.

Parameters

r

-

smart pointer from which ownership will be transferred

Return value

*this

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).

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 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

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