< cpp‎ | memory
Dynamic memory management
Uninitialized storage
Garbage collection support
C Library
Low level memory management
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 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 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


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.

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)
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 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

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

Helper classes

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


#include <iostream>
#include <vector>
#include <memory>
#include <cstdio>
#include <fstream>
#include <cassert>
#include <functional>
struct B {
  virtual void bar() { std::cout << "B::bar\n"; }
  virtual ~B() = default;
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)
    return p;
void close_file(std::FILE* fp) { std::fclose(fp); }
int main()
  std::cout << "unique ownership semantics demo\n";
      auto p = std::make_unique<D>(); // p is a unique_ptr that owns a D
      auto q = pass_through(std::move(p)); 
      assert(!p); // now p owns nothing and holds a null pointer
      q->bar();   // and q owns the D object
  } // ~D called here
  std::cout << "Runtime polymorphism demo\n";
    std::unique_ptr<B> p = std::make_unique<D>(); // p is a unique_ptr that owns a D
                                                  // as a pointer to base
    p->bar(); // virtual dispatch
    std::vector<std::unique_ptr<B>> v;  // unique_ptr can be stored in a container
    v.emplace_back(new D);
    for(auto& p: v) p->bar(); // virtual dispatch
  } // ~D called 3 times
  std::cout << "Custom deleter demo\n";
  std::ofstream("demo.txt") << 'x'; // prepare the file to read
      std::unique_ptr<std::FILE, decltype(&close_file)> fp(std::fopen("demo.txt", "r"),
      if(fp) // fopen could have failed; in which case fp holds a null pointer
        std::cout << (char)std::fgetc(fp.get()) << '\n';
  } // fclose() called here, but only if FILE* is not a null pointer
    // (that is, if fopen succeeded)
  std::cout << "Custom lambda-expression deleter demo\n";
    std::unique_ptr<D, std::function<void(D*)>> p(new D, [](D* ptr)
            std::cout << "destroying from a custom deleter...\n";
            delete ptr;
        });  // p owns D
  } // the lambda above is called and D is destroyed
  std::cout << "Array form of unique_ptr demo\n";
      std::unique_ptr<D[]> p{new D[3]};
  } // calls ~D 3 times


unique ownership semantics demo
Runtime polymorphism demo
Custom deleter demo
Custom lambda-expression deleter demo
destroying from a custom deleter...
Array form of unique_ptr demo