std::unique_ptr<T,Deleter>::operator=
Declarations
- Primary Template, unique_ptr<T>
- Specialization for Arrays, unique_ptr<T[]>
- C++23
- C++11
// 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;
// 1)
unique_ptr& operator=( unique_ptr&& r ) noexcept;
// 2)
template< class U, class E >
unique_ptr& operator=( unique_ptr<U,E>&& r ) noexcept;
// 3)
unique_ptr& operator=( std::nullptr_t ) noexcept;
- C++23
- C++11
// 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;
// 1)
unique_ptr& operator=( unique_ptr&& r ) noexcept;
// 2)
template< class U, class E >
unique_ptr& operator=( unique_ptr<U,E>&& r ) noexcept;
// 3)
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 andunique_ptr<U,E>::pointer
is implicitly convertible to pointer andstd::is_assignable<Deleter&, E&&>::value
istrue
. -
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 aselement_type*
unique_ptr<U,E>::pointer
is the same type asunique_ptr<U,E>::element_type*
unique_ptr<U,E>::element_type(*)[]
is convertible toelement_type(*)[]
std::is_assignable<Deleter&, E&&>::value
istrue
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";
}
Foo 1
Creating new Foo...
Foo 2
~Foo 1
About to leave inner block...
About to leave program...
~Foo 2