std::unique_ptr::reset(3)

NAME
std::unique_ptr::reset - std::unique_ptr::reset

Synopsis
   members of the primary template, unique_ptr<T>
   void reset( pointer ptr = pointer() ) noexcept; (1) (constexpr since C++23)
   members of the specialization unique_ptr<T[]>
   template< class U > (2) (constexpr since C++23)
   void reset( U ptr ) noexcept;
   void reset( std::nullptr_t = nullptr ) noexcept; (3) (constexpr since C++23)
   Replaces the managed object.
   1) Given current_ptr, the pointer that was managed by *this, performs the following
   actions, in this order:
    1. Saves a copy of the current pointer old_ptr = current_ptr
    2. Overwrites the current pointer with the argument current_ptr = ptr
    3. If the old pointer was non-empty, deletes the previously managed object
       if(old_ptr) get_deleter()(old_ptr).
   2) Behaves the same as the reset member of the primary template, except that it will
   only participate in overload resolution if either
    1. U is the same type as pointer, or
    2. pointer is the same type as element_type* and U is a pointer type V* such that
       V(*)[] is convertible to element_type(*)[].
   3) Equivalent to reset(pointer())

Parameters
   ptr - pointer to a new object to manage

Return value
   (none)

Notes
   To replace the managed object while supplying a new deleter as well, move assignment
   operator may be used.
   A test for self-reset, i.e. whether ptr points to an object already managed by
   *this, is not performed, except where provided as a compiler extension or as a
   debugging assert. Note that code such as p.reset(p.release()) does not involve
   self-reset, only code like p.reset(p.get()) does.

Example
// Run this code
 #include <iostream>
 #include <memory>
 struct Foo { // object to manage
     Foo() { std::cout << "Foo...\n"; }
     ~Foo() { std::cout << "~Foo...\n"; }
 };
 struct D { // deleter
     void operator() (Foo* p) {
         std::cout << "Calling delete for Foo object... \n";
         delete p;
     }
 };
 int main()
 {
     std::cout << "Creating new Foo...\n";
     std::unique_ptr<Foo, D> up(new Foo(), D()); // up owns the Foo pointer (deleter D)
     std::cout << "Replace owned Foo with a new Foo...\n";
     up.reset(new Foo()); // calls deleter for the old one
     std::cout << "Release and delete the owned Foo...\n";
     up.reset(nullptr);
 }

Output:
 Creating new Foo...
 Foo...
 Replace owned Foo with a new Foo...
 Foo...
 Calling delete for Foo object...
 ~Foo...
 Release and delete the owned Foo...
 Calling delete for Foo object...
 ~Foo...
  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[]>::reset rejected qualification accepts
                       conversions

See also
   release returns a pointer to the managed object and releases the ownership
           (public member function)