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std::unique_ptr

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< cpp‎ | memory
 
 
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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 and manages another object through a pointer and 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 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 called 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

Contents

[edit] 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. (Conversely, 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.

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

[edit] Non-member functions

creates a unique pointer that manages a new object
(function template) [edit]
compares to another unique_ptr or with nullptr
(function template) [edit]
outputs the value of the managed pointer to an output stream
(function template) [edit]
specializes the std::swap algorithm
(function template) [edit]

[edit] Helper classes

hash support for std::unique_ptr
(class template specialization) [edit]

[edit] Example

#include <cassert>
#include <cstdio>
#include <fstream>
#include <iostream>
#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)
            head = std::move(head->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 p = std::make_unique<D>();
 
        // Transfer ownership to `pass_through`,
        // which in turn transfers ownership back through the return value
        std::unique_ptr 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 instead 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;
        for (int beer = 0; beer != 1'000'000; ++beer)
            wall.push(beer);
 
        std::cout << "1'000'000 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...

[edit] See also

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