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

• Deleter is not MoveAssignable, or
• assigning get_deleter() from an rvalue of type Deleter would throw an exception.

• std::remove_reference<Deleter>::type is not CopyAssignable, or
• assigning get_deleter() from an lvalue of type Deleter would throw an exception.

• 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(*)[].

#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