std::unique_ptr::unique_ptr
From cppreference.com
< cpp | memory | unique ptr
| members of the primary template, unique_ptr<T> |
||
| constexpr unique_ptr(); constexpr unique_ptr( nullptr_t ); |
(1) | |
| explicit unique_ptr( pointer p ); |
(2) | |
| unique_ptr( pointer p, /* see below */ d1 ); |
(3) | |
| unique_ptr( pointer p, /* see below */ d2 ); |
(4) | |
| unique_ptr( unique_ptr&& u ); |
(5) | |
| template< class U, class E > unique_ptr( unique_ptr<U, E>&& u ); |
(6) | |
| template< class U > unique_ptr( auto_ptr<U>&& u ); |
(7) | (until C++17) |
| members of the specialization for arrays, unique_ptr<T[]> |
||
| constexpr unique_ptr(); constexpr unique_ptr( nullptr_t ); |
(1) | |
| explicit unique_ptr( pointer p ); |
(2) | (until C++17) |
| template<class U> explicit unique_ptr( U p ); |
(2) | (since C++17) |
| unique_ptr( pointer p, /* see below */ d1 ); |
(3) | (until C++17) |
| template<class U> unique_ptr( U p, /* see below */ d1 ); |
(3) | (since C++17) |
| unique_ptr( pointer p, /* see below */ d2 ); |
(4) | (until C++17) |
| template<class U> unique_ptr( U p, /* see below */ d2 ); |
(4) | (since C++17) |
| unique_ptr( unique_ptr&& u ); |
(5) | |
| template< class U, class E > unique_ptr( unique_ptr<U, E>&& u ); |
(6) | (since C++17) |
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. |
This constructor is ill-formed if |
(until C++17) |
|
These overloads only participate in overload resolution if std::is_default_constructible<Deleter>::value is true and Deleter is not a pointer type. |
(since C++17) |
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 constructor is ill-formed if |
(until C++17) |
|
This overload only participates in overload resolution if std::is_default_constructible<Deleter>::value is true and Deleter is not a pointer type. |
(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);(requires thatDeleteris nothrow-CopyConstructible) -
unique_ptr(pointer p, A&& d);(requires thatDeleteris nothrow-MoveConstructible)
b) If
D is an lvalue-reference type A&, then the signatures are:
-
unique_ptr(pointer p, A& d); -
unique_ptr(pointer p, A&& d);
c) If
D is an lvalue-reference type const A&, then the signatures are:
-
unique_ptr(pointer p, const A& d); -
unique_ptr(pointer p, const A&& d);
In all cases the deleter is initialized from std::forward<decltype(d)>(d).
|
If |
(until C++17) |
|
If |
(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:
|
(since C++17) |
5) Constructs a
unique_ptr by transferring ownership from u to *this. 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 pointerb) 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
|
(since C++17) |
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 |
Exceptions
noexcept specification:
noexcept
Notes
Instead of using the overload (2) together with new, it is often a 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.
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 } std::cout << "Example constructor(7)...\n"; { std::auto_ptr<Foo> up7a(new Foo); std::unique_ptr<Foo> up7b(std::move(up7a)); // ownership transfer } }
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