std::unique_ptr

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C Library
Low level memory management
 
std::unique_ptr
 
Defined in header <memory>
template<

    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 also own no objects, 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

Contents

[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.)

[edit] Member types

Member type Definition
pointer std::remove_reference<Deleter>::type::pointer if that type exists, otherwise T*
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)
Modifiers
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)
Observers
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

(C++14)
 creates a unique pointer that manages a new object
(function template)
compares to another unique_ptr or with nullptr
(function template)
(C++11)
 specializes the std::swap algorithm
(function template)

[edit] Helper classes

(C++11)
 hash support for std::unique_ptr
(class template specialization)

[edit] Example

Run this code
#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
        f(*p2);
 
        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
}

Output: 

Foo::Foo
Foo::bar
f(const Foo&)
destroying p2...
Foo::bar
Foo::~Foo
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