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

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) noexcept;

(1) (requires that Deleter is nothrow-CopyConstructible)

unique_ptr(pointer p, A&& d) noexcept;

(2) (requires that Deleter is 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 deduction.

(since C++17)

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 Deleter, or Deleter is not a reference type and E is implicitly convertible to Deleter.

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>.

#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

[edit] 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 qualification conversions.	Accept.
LWG 2520	C++11	`unique_ptr<T[]>` was accidentally made non-constructible from `nullptr_t`.	Made constructible.
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 and a deleter was wrong.	Corrected.
LWG 2944	C++11	Some preconditions were accidentally dropped by LWG 2905	Restored.

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

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