< cpp‎ | memory
Defined in header <memory>

    class T,
    class Deleter = std::default_delete<T>

> class unique_ptr;
(1) (since C++11)
template <

    class T,
    class Deleter

> class unique_ptr<T[], Deleter>;
(2) (since C++11)

std::unique_ptr is a smart pointer that retains sole ownership of an object through a pointer and destroys that object when the unique_ptr goes out of scope. No two unique_ptr instances can manage the same object.

The object is destroyed and its memory deallocated when either of the following happens:

  • unique_ptr managing the object is destroyed
  • unique_ptr managing the object is assigned another pointer via operator= or reset().

The object is destroyed using a potentially user-supplied deleter by calling Deleter(ptr). The deleter calls the destructor of the object and dispenses the memory.

A unique_ptr may alternatively own no object, in which case it is called empty.

There are two versions of std::unique_ptr:

1) Manages the lifetime of a single object (e.g. allocated with new)
2) Manages the lifetime of a dynamically-allocated array of objects (e.g. allocated with new[])

The class satisfies the requirements of MoveConstructible and MoveAssignable, but not the requirements of either CopyConstructible or CopyAssignable.

Type requirements
Deleter must be FunctionObject or lvalue reference to a FunctionObject or lvalue reference to function, callable with an argument of type unique_ptr<T, Deleter>::pointer


[edit] Notes

Only non-const unique_ptr can transfer the ownership of the managed object to another unique_ptr. The lifetime of an object managed by const std::unique_ptr is limited to the scope in which the pointer was created.

Typical uses of std::unique_ptr include:

  • providing exception safety to classes and functions that handle objects with dynamic lifetime, by guaranteeing deletion on both normal exit and exit through exception
  • passing ownership of uniquely-owned objects with dynamic lifetime into functions
  • acquiring ownership of uniquely-owned objects with dynamic lifetime from functions
  • as the element type in move-aware containers, such as std::vector, which hold pointers to dynamically-allocated objects (e.g. if polymorphic behavior is desired)

std::unique_ptr may be constructed for an incomplete type T, such as to facilitate the use as a handle in the Pimpl idiom. If the default deleter is used, T must be complete at the point in code where the deleter is invoked, which happens in the destructor, move assignment operator, and reset member function of std::unique_ptr. (Conversely, std::shared_ptr can't be constructed from a raw pointer to incomplete type, but can be destroyed where T is incomplete).

If T is a derived class of some base B, then std::unique_ptr<T> is implicitly convertible to std::unique_ptr<B>. The default deleter of the resulting std::unique_ptr<B> will use operator delete for B, leading to undefined behavior unless the destructor of B is virtual. Note that std::shared_ptr behaves differently: std::shared_ptr<B> will use the operator delete for the type T and the owned object will be deleted correctly even if the destructor of B is not virtual.

Unlike std::shared_ptr, std::unique_ptr may manage an object through any custom handle type that satisfies NullablePointer. This allows, for example, managing objects located in shared memory, by supplying a Deleter that defines typedef boost::offset_ptr pointer;

[edit] Member types

Member type Definition
pointer std::remove_reference<Deleter>::type::pointer if that type exists, otherwise T*. Must satisfy NullablePointer
element_type T, the type of the object managed by this unique_ptr
deleter_type Deleter, the function object or lvalue reference to function or to function object, to be called from the destructor

[edit] Member functions

constructs a new unique_ptr
(public member function)
destructs the managed object if such is present
(public member function)
assigns the unique_ptr
(public member function)
returns a pointer to the managed object and releases the ownership
(public member function)
replaces the managed object
(public member function)
swaps the managed objects
(public member function)
returns a pointer to the managed object
(public member function)
returns the deleter that is used for destruction of the managed object
(public member function)
checks if there is associated managed object
(public member function)
Single-object version, unique_ptr<T>
dereferences pointer to the managed object
(public member function)
Array version, unique_ptr<T[]>
provides indexed access to the managed array
(public member function)

[edit] Non-member functions

creates a unique pointer that manages a new object
(function template)
compares to another unique_ptr or with nullptr
(function template)
specializes the std::swap algorithm
(function template)

[edit] Helper classes

hash support for std::unique_ptr
(class template specialization)

[edit] Example

#include <iostream>
#include <memory>
struct Foo
    Foo()      { std::cout << "Foo::Foo\n";  }
    ~Foo()     { std::cout << "Foo::~Foo\n"; }
    void bar() { std::cout << "Foo::bar\n";  }
void f(const Foo &)
    std::cout << "f(const Foo&)\n";
int main()
    std::unique_ptr<Foo> p1(new Foo);  // p1 owns Foo
    if (p1) p1->bar();
        std::unique_ptr<Foo> p2(std::move(p1));  // now p2 owns Foo
        p1 = std::move(p2);  // ownership returns to p1
        std::cout << "destroying p2...\n";
    if (p1) p1->bar();
    // Foo instance is destroyed when p1 goes out of scope


f(const Foo&)
destroying p2...