std::unique_ptr

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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 owns (is responsible for) and manages another object via a pointer and subsequently disposes of that object when the unique_ptr goes out of scope.

The object is disposed of, using the associated deleter, when either of the following happens:

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

The object is disposed of, using a potentially user-supplied deleter, by calling get_deleter()(ptr). The default deleter (std::default_delete) uses the delete operator, which destroys the object and deallocates the memory.

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

There are two versions of std::unique_ptr:

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

The class satisfies the requirements of MoveConstructible and MoveAssignable, but of neither CopyConstructible nor 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.

Notes

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

std::unique_ptr is commonly used to manage the lifetime of objects, including:

  • 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. (In contrast, std::shared_ptr can't be constructed from a raw pointer to incomplete type, but can be destroyed where T is incomplete). Note that if T is a class template specialization, use of unique_ptr as an operand, e.g. !p requires T's parameters to be complete due to ADL.

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; or another fancy pointer.

Feature-test macro Value Std Feature
__cpp_lib_constexpr_memory 202202L (C++23) constexpr std::unique_ptr

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

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 an 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)

Non-member functions

(C++14)(C++20)
 creates a unique pointer that manages a new object
(function template)
(removed in C++20)(C++20)
 compares to another unique_ptr or with nullptr
(function template)
(C++20)
 outputs the value of the managed pointer to an output stream
(function template)
(C++11)
 specializes the std::swap algorithm
(function template)

Helper classes

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

Example

Run this code
#include <cassert>
#include <cstdio>
#include <fstream>
#include <iostream>
#include <locale>
#include <memory>
#include <stdexcept>
 
// helper class for runtime polymorphism demo below
struct B
{
    virtual ~B() = default;
 
    virtual void bar() { std::cout << "B::bar\n"; }
};
 
struct D : B
{
    D() { std::cout << "D::D\n"; }
    ~D() { std::cout << "D::~D\n"; }
 
    void bar() override { std::cout << "D::bar\n"; }
};
 
// a function consuming a unique_ptr can take it by value or by rvalue reference
std::unique_ptr<D> pass_through(std::unique_ptr<D> p)
{
    p->bar();
    return p;
}
 
// helper function for the custom deleter demo below
void close_file(std::FILE* fp)
{
    std::fclose(fp);
}
 
// unique_ptr-based linked list demo
struct List
{
    struct Node
    {
        int data;
        std::unique_ptr<Node> next;
    };
 
    std::unique_ptr<Node> head;
 
    ~List()
    {
        // destroy list nodes sequentially in a loop, the default destructor
        // would have invoked its `next`'s destructor recursively, which would
        // cause stack overflow for sufficiently large lists.
        while (head)
        {
            auto next = std::move(head->next);
            head = std::move(next);
        }
    }
 
    void push(int data)
    {
        head = std::unique_ptr<Node>(new Node{data, std::move(head)});
    }
};
 
int main()
{
    std::cout << "1) Unique ownership semantics demo\n";
    {
        // Create a (uniquely owned) resource
        std::unique_ptr<D> p = std::make_unique<D>();
 
        // Transfer ownership to `pass_through`,
        // which in turn transfers ownership back through the return value
        std::unique_ptr<D> q = pass_through(std::move(p));
 
        // p is now in a moved-from 'empty' state, equal to nullptr
        assert(!p);
    }
 
    std::cout << "\n" "2) Runtime polymorphism demo\n";
    {
        // Create a derived resource and point to it via base type
        std::unique_ptr<B> p = std::make_unique<D>();
 
        // Dynamic dispatch works as expected
        p->bar();
    }
 
    std::cout << "\n" "3) Custom deleter demo\n";
    std::ofstream("demo.txt") << 'x'; // prepare the file to read
    {
        using unique_file_t = std::unique_ptr<std::FILE, decltype(&close_file)>;
        unique_file_t fp(std::fopen("demo.txt", "r"), &close_file);
        if (fp)
            std::cout << char(std::fgetc(fp.get())) << '\n';
    } // `close_file()` called here (if `fp` is not null)
 
    std::cout << "\n" "4) Custom lambda-expression deleter and exception safety demo\n";
    try
    {
        std::unique_ptr<D, void(*)(D*)> p(new D, [](D* ptr)
        {
            std::cout << "destroying from a custom deleter...\n";
            delete ptr;
        });
 
        throw std::runtime_error(""); // `p` would leak here if it were a plain pointer
    }
    catch (const std::exception&)
    {
        std::cout << "Caught exception\n";
    }
 
    std::cout << "\n" "5) Array form of unique_ptr demo\n";
    {
        std::unique_ptr<D[]> p(new D[3]);
    } // `D::~D()` is called 3 times
 
    std::cout << "\n" "6) Linked list demo\n";
    {
        List wall;
        const int enough{1'000'000};
        for (int beer = 0; beer != enough; ++beer)
            wall.push(beer);
 
        std::cout.imbue(std::locale("en_US.UTF-8"));
        std::cout << enough << " bottles of beer on the wall...\n";
    } // destroys all the beers
}

Possible output: 

1) Unique ownership semantics demo
D::D
D::bar
D::~D
 
2) Runtime polymorphism demo
D::D
D::bar
D::~D
 
3) Custom deleter demo
x
 
4) Custom lambda-expression deleter and exception safety demo
D::D
destroying from a custom deleter...
D::~D
Caught exception
 
5) Array form of unique_ptr demo
D::D
D::D
D::D
D::~D
D::~D
D::~D
 
6) Linked list demo
1,000,000 bottles of beer on the wall...

See also

(C++11)
 smart pointer with shared object ownership semantics
(class template)
(C++11)
 weak reference to an object managed by std::shared_ptr
(class template)
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