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std::unique...::unique_ptr(3) C++ Standard Libarystd::unique...::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) (const- expr since C++23) unique_ptr( pointer p, /* see below */ d1 ) noexcept; (3) (const- expr since C++23) unique_ptr( pointer p, /* see below */ d2 ) noexcept; (4) (const- expr since C++23) unique_ptr( unique_ptr&& u ) noexcept; (5) (const- expr since C++23) template< class U, class E > (6) (const- expr since C++23) unique_ptr( unique_ptr<U, E>&& u ) noexcept; template< class U > (7) (re- moved 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) (const- expr since C++23) template< class U > unique_ptr( U p, /* see below */ d1 (3) (const- expr since C++23) ) noexcept; template< class U > unique_ptr( U p, /* see below */ d2 (4) (const- expr since C++23) ) noexcept; unique_ptr( unique_ptr&& u ) noexcept; (5) (const- expr since C++23) template< class U, class E > (6) (const- expr 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 DefaultCon- structible and that construction does not throw an exception. These overloads par- ticipate in overload resolution only if std::is_default_con- structible<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 excep- tion. 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 deduc- tion. (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<de- cltype(d)>(d). These overloads participate in overload resolution only if std::is_con- structible<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 con- structors that take a pointer parameter in the primary template except that they addi- tionally 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 pri- mary 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 ele- ment_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::de- fault_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-lo- cal 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 ar- ray (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 ref- erence 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 trans- fer } 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 retroac- tively to previously published C++ standards. DR Applied to Behavior as published Correct behavior LWG 2118 C++11 Constructors of unique_ptr<T[]> rejected Ac- cept. 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 Re- stored. by LWG 2905 http://cppreference.com 2022.07.31 std::unique...::unique_ptr(3)
NAME | Synopsis | Parameters | Notes | Example | Output:
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