std::unique_ptr::operator=(3)

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

Synopsis
   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)
   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 Behavior as published Correct behavior
          to
   LWG for overload (2), get_deleter() was corrected to
   2047 C++11 assigned from std::forward<E>(r.get_deleter())
                std::forward<Deleter>(r.get_deleter())
   LWG C++11 unique_ptr<T[]>::operator= accepts
   2118 rejected qualification conversions
   LWG C++11 the converting assignment operator was constrained
   2228 not constrained
   LWG C++11 the move assignment operator was not constrained
   2899 constrained