std::unique_ptr::unique_ptr(3)

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

Synopsis
   members of the primary template, unique_ptr<T>
   constexpr unique_ptr() noexcept; (1)
   constexpr unique_ptr( std::nullptr_t ) noexcept;
   explicit unique_ptr( pointer p ) noexcept; (2) (constexpr since C++23)
   unique_ptr( pointer p, /* see below */ d1 ) noexcept; (3) (constexpr since C++23)
   unique_ptr( pointer p, /* see below */ d2 ) noexcept; (4) (constexpr since C++23)
   unique_ptr( unique_ptr&& u ) noexcept; (5) (constexpr since C++23)
   template< class U, class E > (6) (constexpr since C++23)
   unique_ptr( unique_ptr<U, E>&& u ) noexcept;
   unique_ptr( const unique_ptr& ) = delete; (7)
   template< class U > (8) (removed in C++17)
   unique_ptr( std::auto_ptr<U>&& u ) noexcept;
   members of the specialization for arrays,
   unique_ptr<T[]>
   constexpr unique_ptr() noexcept; (1)
   constexpr unique_ptr( std::nullptr_t ) noexcept;
   template< class U > explicit unique_ptr( U p ) noexcept; (2) (constexpr since C++23)
   template< class U > unique_ptr( U p, /* see below */ d1 (3) (constexpr since C++23)
   ) noexcept;
   template< class U > unique_ptr( U p, /* see below */ d2 (4) (constexpr since C++23)
   ) noexcept;
   unique_ptr( unique_ptr&& u ) noexcept; (5) (constexpr since C++23)
   template< class U, class E > (6) (constexpr since C++23)
   unique_ptr( unique_ptr<U, E>&& u ) noexcept;
   unique_ptr( const unique_ptr& ) = delete; (7)
   1) Constructs a std::unique_ptr that owns nothing. Value-initializes the stored
   pointer and the stored deleter. Requires that Deleter is DefaultConstructible and
   that construction does not throw an exception. These overloads participate in
   overload resolution only if std::is_default_constructible<Deleter>::value is true
   and Deleter is not a pointer type.
   2) Constructs a std::unique_ptr which owns p, initializing the stored pointer with p
   and value-initializing the stored deleter. Requires that Deleter is
   DefaultConstructible and that construction does not throw an exception. This
   overload participates in overload resolution only if
   std::is_default_constructible<Deleter>::value is true and Deleter is not a pointer
   type.
   This constructor is not selected by class template argument deduction. (since C++17)
   3,4) Constructs a std::unique_ptr object which owns p, initializing the stored
   pointer with p and initializing a deleter D as below (depends upon whether D is a
   reference type).
   a) If D is non-reference type A, then the signatures are:
   unique_ptr(pointer p, const A& d) (1) (requires that Deleter is
   noexcept; nothrow-CopyConstructible)
   unique_ptr(pointer p, A&& d) (2) (requires that Deleter is
   noexcept; nothrow-MoveConstructible)
   b) If D is an lvalue-reference type A&, then the signatures are:
   unique_ptr(pointer p, A& d) noexcept; (1)
   unique_ptr(pointer p, A&& d) = delete; (2)
   c) If D is an lvalue-reference type const A&, then the signatures are:
   unique_ptr(pointer p, const A& d) noexcept; (1)
   unique_ptr(pointer p, const A&& d) = delete; (2)
   In all cases the deleter is initialized from std::forward<decltype(d)>(d). These
   overloads participate in overload resolution only if std::is_constructible<D,
   decltype(d)>::value is true.
   These two constructors are not selected by class template argument (since C++17)
   deduction.
   2-4) In the specialization for arrays behave the same as the constructors that take
   a pointer parameter in the primary template except that they additionally do not
   participate in overload resolution unless one of the following is true:
     * U is the same type as pointer, or
     * U is std::nullptr_t, or
     * pointer is the same type as element_type* and U is some pointer type V* such
       that V(*)[] is implicitly convertible to element_type(*)[].
   5) Constructs a unique_ptr by transferring ownership from u to *this and stores the
   null pointer in u. This constructor only participates in overload resolution if
   std::is_move_constructible<Deleter>::value is true. If Deleter is not a reference
   type, requires that it is nothrow-MoveConstructible (if Deleter is a reference,
   get_deleter() and u.get_deleter() after move construction reference the same value).
   6) Constructs a unique_ptr by transferring ownership from u to *this, where u is
   constructed with a specified deleter (E). It depends upon whether E is a reference
   type, as following:
   a) if E is a reference type, this deleter is copy constructed from u's deleter
   (requires that this construction does not throw),
   b) if E is a non-reference type, this deleter is move constructed from u's deleter
   (requires that this construction does not throw).
   This constructor only participates in overload resolution if all of the following is
   true:
   a) unique_ptr<U, E>::pointer is implicitly convertible to pointer,
   b) U is not an array type,
   c) either Deleter is a reference type and E is the same type as D, or Deleter is not
   a reference type and E is implicitly convertible to D.
   6) In the specialization for arrays behaves the same as in the primary template,
   except that it 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(*)[],
     * either Deleter is a reference type and E is the same type as Deleter, or Deleter
       is not a reference type and E is implicitly convertible to Deleter.
   7) Copy constructor is explicitly deleted.
   8) Constructs a unique_ptr where the stored pointer is initialized with u.release()
   and the stored deleter is value-initialized. This constructor only participates in
   overload resolution if U* is implicitly convertible to T* and Deleter is the same
   type as std::default_delete<T>.

Parameters
   p - a pointer to an object to manage
   d1, d2 - a deleter to use to destroy the object
   u - another smart pointer to acquire the ownership from

Notes
   Instead of using the overload (2) together with new, it is often a (since C++14)
   better idea to use std::make_unique<T>.
   std::unique_ptr<Derived> is implicitly convertible to std::unique_ptr<Base> through
   the overload (6) (because both the managed pointer and std::default_delete are
   implicitly convertible).
   Because the default constructor is constexpr, static unique_ptrs are initialized as
   part of static non-local initialization, before any dynamic non-local initialization
   begins. This makes it safe to use a unique_ptr in a constructor of any static
   object.
   There is no class template argument deduction from pointer type
   because it is impossible to distinguish a pointer obtained from array (since C++17)
   and non-array forms of new.

Example
// Run this code
 #include <iostream>
 #include <memory>
 struct Foo // object to manage
 {
     Foo() { std::cout << "Foo ctor\n"; }
     Foo(const Foo&) { std::cout << "Foo copy ctor\n"; }
     Foo(Foo&&) { std::cout << "Foo move ctor\n"; }
     ~Foo() { std::cout << "~Foo dtor\n"; }
 };
 struct D // deleter
 {
     D() {};
     D(const D&) { std::cout << "D copy ctor\n"; }
     D(D&) { std::cout << "D non-const copy ctor\n"; }
     D(D&&) { std::cout << "D move ctor \n"; }
     void operator()(Foo* p) const
     {
         std::cout << "D is deleting a Foo\n";
         delete p;
     };
 };
 int main()
 {
     std::cout << "Example constructor(1)...\n";
     std::unique_ptr<Foo> up1; // up1 is empty
     std::unique_ptr<Foo> up1b(nullptr); // up1b is empty
     std::cout << "Example constructor(2)...\n";
     {
         std::unique_ptr<Foo> up2(new Foo); //up2 now owns a Foo
     } // Foo deleted
     std::cout << "Example constructor(3)...\n";
     D d;
     { // deleter type is not a reference
         std::unique_ptr<Foo, D> up3(new Foo, d); // deleter copied
     }
     { // deleter type is a reference
         std::unique_ptr<Foo, D&> up3b(new Foo, d); // up3b holds a reference to d
     }
     std::cout << "Example constructor(4)...\n";
     { // deleter is not a reference
         std::unique_ptr<Foo, D> up4(new Foo, D()); // deleter moved
     }
     std::cout << "Example constructor(5)...\n";
     {
         std::unique_ptr<Foo> up5a(new Foo);
         std::unique_ptr<Foo> up5b(std::move(up5a)); // ownership transfer
     }
     std::cout << "Example constructor(6)...\n";
     {
         std::unique_ptr<Foo, D> up6a(new Foo, d); // D is copied
         std::unique_ptr<Foo, D> up6b(std::move(up6a)); // D is moved
         std::unique_ptr<Foo, D&> up6c(new Foo, d); // D is a reference
         std::unique_ptr<Foo, D> up6d(std::move(up6c)); // D is copied
     }
 #if (__cplusplus < 201703L)
     std::cout << "Example constructor(7)...\n";
     {
         std::auto_ptr<Foo> up7a(new Foo);
         std::unique_ptr<Foo> up7b(std::move(up7a)); // ownership transfer
     }
 #endif
     std::cout << "Example array constructor...\n";
     {
         std::unique_ptr<Foo[]> up(new Foo[3]);
     } // three Foo objects deleted
 }

Output:
 Example constructor(1)...
 Example constructor(2)...
 Foo ctor
 ~Foo dtor
 Example constructor(3)...
 Foo ctor
 D copy ctor
 D is deleting a Foo
 ~Foo dtor
 Foo ctor
 D is deleting a Foo
 ~Foo dtor
 Example constructor(4)...
 Foo ctor
 D move ctor
 D is deleting a Foo
 ~Foo dtor
 Example constructor(5)...
 Foo ctor
 ~Foo dtor
 Example constructor(6)...
 Foo ctor
 D copy ctor
 D move ctor
 Foo ctor
 D non-const copy ctor
 D is deleting a Foo
 ~Foo dtor
 D is deleting a Foo
 ~Foo dtor
 Example constructor(7)...
 Foo ctor
 ~Foo dtor
 Example array constructor...
 Foo ctor
 Foo ctor
 Foo ctor
 ~Foo dtor
 ~Foo dtor
 ~Foo dtor
   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 Constructors of unique_ptr<T[]> rejected Accept.
                       qualification conversions.
   LWG 2520 C++11 unique_ptr<T[]> was accidentally made Made constructible.
                       non-constructible from nullptr_t.
   LWG 2801 C++11 The default constructor was not constrained. Constrained.
   LWG 2899 C++11 The move constructor was not constrained. Constrained.
   LWG 2905 C++11 Constraint on the constructor from a pointer Corrected.
                       and a deleter was wrong.
   LWG 2944 C++11 Some preconditions were accidentally dropped Restored.
                       by LWG 2905