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

Declarations​

// 1)
constexpr unique_ptr& operator=( unique_ptr&& r ) noexcept;
// 2)
template< class U, class E >
constexpr unique_ptr& operator=( unique_ptr<U,E>&& r ) noexcept;
// 3)
constexpr unique_ptr& operator=( std::nullptr_t ) noexcept;

Description​

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​

#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";
}

Result
Foo 1
Creating new Foo...
Foo 2
~Foo 1
About to leave inner block...
About to leave program...
~Foo 2