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std::unique_ptr::operator=(3) | C++ Standard Libary | std::unique_ptr::operator=(3) |
NAME¶
std::unique_ptr::operator= - std::unique_ptr::operator=
Synopsis¶
members of the primary template, unique_ptr<T>
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)
members of the specialization for arrays,
unique_ptr<T[]>
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)
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¶
// 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 to Behavior as published Correct behavior
LWG 2118 C++11 unique_ptr<T[]>::operator= rejected accepts
qualification conversions
LWG 2228 C++11 the converting assignment operator was not constrained
constrained
LWG 2899 C++11 the move assignment operator was not constrained
constrained
2022.07.31 | http://cppreference.com |