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std::unique_ptr::operator=

members of the primary template, unique_ptr<T>
unique_ptr& operator=( unique_ptr&& r ) noexcept;	(1)
template< class U, class E > unique_ptr& operator=( unique_ptr<U,E>&& r ) noexcept;	(1)
unique_ptr& operator=( nullptr_t ) noexcept;	(2)
members of the specialization for arrays, unique_ptr<T[]>
unique_ptr& operator=( unique_ptr&& r ) noexcept;	(1)
template< class U, class E > unique_ptr& operator=( unique_ptr<U,E>&& r ) noexcept;	(1)	(since C++17)
unique_ptr& operator=( nullptr_t ) noexcept;	(2)

1) Transfers ownership from r to *this as if by calling reset(r.release()) followed by an assignment of get_deleter() from std::forward<E>(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 template version of this assignment operator 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 (since C++17).

The template version of 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

(since C++17)

2) 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

smart pointer from which ownership will be transfered

Return value

*this.

Example

#include <iostream>
#include <memory>
 
struct Foo {
    Foo() { std::cout << "Foo\n"; }
    ~Foo() { std::cout << "~Foo\n"; }
};
 
int main() 
{
    std::unique_ptr<Foo> p1;
 
    {
        std::cout << "Creating new Foo...\n";
        std::unique_ptr<Foo> p2( std::make_unique<Foo>() );
        // 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:

Creating new Foo...
Foo
About to leave inner block...
About to leave program...
~Foo

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