std::unique_ptr
| Defined in header <memory>
|
||
| template< class T, |
(1) | (since C++11) |
| template < class T, |
(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_ptrobject is destroyed - the managing
unique_ptrobject 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:
- Manages a single object (e.g. allocated with new)
- 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. (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.
| Feature-test macro | Value | Std | Comment |
|---|---|---|---|
__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,
| |
| dereferences pointer to the managed object (public member function) | |
Array version,
| |
| 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
#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) |