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;
template< class U > (7) (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;

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) 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