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# std::unique_ptr::operator=

From cppreference.com
< cpp‎ | memory‎ | unique ptr

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

### [edit] Parameters

r | - | smart pointer from which ownership will be transfered

### [edit] Return value

*this

### [edit] Exceptions

`noexcept` specification:
noexcept

### [edit] Example

Run this code

    #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(new 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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