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# std::unique_ptr

From cppreference.com
< cpp‎ | memory

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 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`. 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

(constructor) | constructs a new `unique_ptr` (public member function) [edit]
(destructor) | destructs the managed object if such is present (public member function) [edit]
operator= | assigns the `unique_ptr` (public member function) [edit]
##### Modifiers
release | returns a pointer to the managed object and releases the ownership (public member function) [edit]
reset | replaces the managed object (public member function) [edit]
swap | swaps the managed objects (public member function) [edit]
##### Observers
get | returns a pointer to the managed object (public member function) [edit]
get_deleter | returns the deleter that is used for destruction of the managed object (public member function) [edit]
operator bool | checks if there is an associated managed object (public member function) [edit]
##### Single-object version, `unique_ptr<T>`
operator*operator-> | dereferences pointer to the managed object (public member function) [edit]
##### Array version, `unique_ptr<T[]>`
operator[] | provides indexed access to the managed array (public member function) [edit]

### [edit] Non-member functions

make_unique (C++14) | creates a unique pointer that manages a new object (function template) [edit]
operator==operator!=operator<operator<=operator>operator>= | compares to another `unique_ptr` or with nullptr (function template) [edit]
std::swap(std::unique_ptr) (C++11) | specializes the std::swap algorithm (function template) [edit]

### [edit] Helper classes

std::hash<std::unique_ptr> (C++11) | hash support for std::unique_ptr (class template specialization) [edit]

### [edit] Example

Run this code

    #include <iostream>
    #include <vector>
    #include <memory>
    #include <cstdio>
    #include <fstream>
    #include <cassert>

    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)
    {
        p->bar();
        return p;
    }

    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.push_back(std::make_unique<D>());
        v.push_back(std::move(p));
        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(&std::fclose)> fp(std::fopen("demo.txt", "r"),
                                                                &std::fclose);
          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
        p->bar();
      } // 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
    }

Output:

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

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