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std::unique_ptr<T,Deleter>::operator= - cppreference.com

unique_ptr& operator=( unique_ptr&& r ) noexcept;

template< class U, class E >
unique_ptr& operator=( unique_ptr<U, E>&& r ) noexcept;

unique_ptr& operator=( const unique_ptr& ) = delete;

Move assignment operator. Transfers ownership from

to

as if by calling

followed by assigning

from

.

If

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 (

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.

Converting assignment operator. Transfers ownership from

to

as if by calling

followed by assigning

from

.

This overload participates in overload resolution only if all following conditions are satisfied:

• std::is_assignable<Deleter&, E&&>::value is true.
• For the primary template, all following conditions are satisfied:
  • U is not an array type.
  • unique_ptr<U, E>::pointer is implicitly convertible to pointer, and.
• For the array specialization (unique_ptr<T[]>), all following conditions are satisfied:
  • 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(*)[].

If

is not a reference type, the behavior is undefined if assigning

from an

of type

is ill-formed or would throw an exception.

Otherwise (

is a reference type), the behavior is undefined if assigning

from an

of type

is ill-formed or would throw an exception.

Effectively the same as calling

.

4) Copy assignment operator is explicitly deleted.

*this

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).

#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

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

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