(until C++23) | ||||
(until C++23) | ||||
(until C++23) | ||||
(until C++23) | ||||
(until C++23) | ||||
(until C++23) | ||||
) | ||||
) | ||||
) |
start_lifetime_as_array (C++23) | ||||
) | ||||
unique_ptr::operator-> | ||||
make_unique_for_overwrite (C++20) | ||||
operator!=operator<operator>operator<=operator>=operator<=> (until C++20)(C++20) | ||||
operator=( unique_ptr&& r ) noexcept; | (1) | (constexpr since C++23) |
< class U, class E > unique_ptr& operator=( unique_ptr<U, E>&& r ) noexcept; | (2) | (constexpr since C++23) |
operator=( ) noexcept; | (3) | (constexpr since C++23) |
operator=( const unique_ptr& ) = delete; | (4) | |
Parameters Return value Notes Example Defect reports |
r | - | smart pointer from which ownership will be transferred |
[ edit ] notes.
As a move-only type, unique_ptr 's assignment operator only accepts rvalues arguments (e.g. the result of std::make_unique or a std::move 'd unique_ptr variable).
[ edit ] defect reports.
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR | Applied to | Behavior as published | Correct behavior |
---|---|---|---|
C++11 | for overload (2), was assigned from <Deleter>(r.get_deleter()) | corrected to <E>(r.get_deleter()) | |
C++11 | rejected qualification conversions | accepts | |
C++11 | the converting assignment operator was not constrained | constrained | |
C++11 | the move assignment operator was not constrained | constrained |
std::unique_ptr is a smart pointer introduced in C++11. It automatically manages the dynamically allocated resources on the heap. Smart pointers are just wrappers around regular old pointers that help you prevent widespread bugs. Namely, forgetting to delete a pointer and causing a memory leak or accidentally deleting a pointer twice or in the wrong way. They can be used in a similar way to standard pointers. They automate some of the manual processes that cause common bugs.
Prerequisites: Pointer in C++ , Smart Pointers in C++.
When we write unique_ptr<A> ptr1 (new A), memory is allocated on the heap for an instance of datatype A. ptr1 is initialized and points to newly created A object. Here, ptr1 is the only owner of the newly created object A and it manages this object’s lifetime. This means that when ptr1 is reset or goes out of scope, memory is automatically deallocated and A’s object is destroyed.
When ownership of resource is required. When we want single or exclusive ownership of a resource, then we should go for unique pointers. Only one unique pointer can point to one resource. So, one unique pointer cannot be copied to another. Also, it facilitates automatic cleanup when dynamically allocated objects go out of scope and helps preventing memory leaks.
Note: We need to use the <memory> header file for using these smart pointers.
Lets create a structure A and it will have a method named printA to display some text. Then in the main section, let’s create a unique pointer that will point to the structure A. So at this point, we have an instance of structure A and p1 holds the pointer to that.
Now let’s create another pointer p2 and we will try to copy the pointer p1 using the assignment operator(=).
The above code will give compile time error as we cannot assign pointer p2 to p1 in case of unique pointers. We have to use the move semantics for such purpose as shown below.
Managing object of type A using move semantics.
Note once the address in pointer p1 is copied to pointer p2, the pointer p1’s address becomes NULL(0) and the address stored by p2 is now the same as the address stored by p1 showing that the address in p1 has been transferred to the pointer p2 using the move semantics.
Similar reads.
Programming Tutorials
When working with unique_ptr in C++, it’s essential to know how to renounce the ownership of the managed object and free the associated memory when necessary. The unique_ptr class provides the reset() member function for this purpose.
Calling the reset() method on a unique_ptr will perform two operations:
Let’s explore how this works with an example using an int as the managed object for simplicity:
After you have called reset() , it’s a good practice to check whether the unique_ptr is indeed empty (i.e., it now points to nullptr ). You can do this by directly using the unique_ptr in a condition as it has an operator bool defined which checks for ownership of an object:
Resetting a unique_ptr can be useful in several scenarios:
The ability to reset a unique_ptr is an integral part of its functionality, providing you with control over when to free the managed memory. It also helps in preventing memory leaks by ensuring that memory is not inadvertently retained. WRemember, once reset, always check the unique_ptr before using it to avoid accessing nullptr .
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I have a problem very similar to
How to make a class with a member of unique_ptr work with std::move and std::swap?
and I am using VS2013 so I expect the following to work.
path\engine.cpp(75): error C2280: 'std::unique_ptr> &std::unique_ptr<_Ty,std::default_delete<_Ty>>::operator =(const std::unique_ptr<_Ty,std::default_delete<_Ty>> &)' : attempting to reference a deleted function with [ _Ty=AbstractCamera ] other_path\memory(1487) : see declaration of 'std::unique_ptr>::operator =' with [ _Ty=AbstractCamera ] This diagnostic occurred in the compiler generated function 'RenderManager &RenderManager::operator =(const RenderManager &)'
I do get that simple examples like
are not allowed but my problem is the following code:
because the assignment operator of unique_ptr is deleted but how does this affect a class hierarchy like those of Engine->RenderManger->unique_ptr member.
I do get, that Engine e uses the default constructor Engine() and e = Engine() calls the default constructor and the operator= of Engine to assign the temporary Engine object to e.
My question therefore is: where does the code try to copy/assign unique_ptr and how do I resolve it?
I tried to strip the original code down as much as possible but was not able to reproduce the error using a simpler example and ideone in form of a SSCCEE as I do not really understand what causes the problem, so sorry for that!
VS2013 does not automatically generate move special member functions (this is non-conforming, in case it isn't obvious). You'll have to write the move constructor and move assignment operator of Engine and RenderManager yourself, as otherwise the copy special member functions will be used.
As a side note, _currentCamera(std::move(std::make_unique<CameraImpl>)) won't compile; _currentCamera(std::move(std::make_unique<CameraImpl>())) would but the std::move is useless and a pessimization. make_unique already returns an rvalue. There are also several other typos in your code, presumably caused by the minimization process.
T.C. correctly answered the question, but I wanted to give some further insights as I did actually asked the wrong question :)
The real question therefore should have been
How can I use the following code
when my intent is the destruction of the default constructed object e and the assignment of the newly constructed object.
First of all one needs to understand the difference between copy and move assignment and construction and the following thread and especially the two long posts by @FredOverflow here can help very much to understand this: What are move semantics?
The problem setting are actually classes with std::vectors<std::unique_ptr<T>> and classes with std::unique_ptr<T> members. See http://www.cplusplus.com/forum/beginner/110610/ and the answer of JLJorges for a longer example.
JLJorges posted:
So don't do anything: by default B is not Copyable or Assignable, but B is Moveable and MoveAssignable
JLJorges pointed out two important things under such circumstances:
VS2013 therefore is able to implicitly generate move and move assign operators/constructors as it does exactly this for such a class setup.
The reason I did not select T.C.'s answer altough he was very helpful and close to the answer is the following:
In case one needs copy-able/copy-assignable classes he has to implement copy-constructor and copy-assignment . This would have been the answer if I would have needed exactly what the code in the initial question looked like: a copy/copy-assignable class Engine.
In case one implements one of those methods/operators he might want to implement the others as well, see the following links for explanations:
As I said, I had the asked the wrong question, as I did not needed a copyable/copyassignable class the solution would have been to implement move constructor and move assign operator or like JLJorges pointed out: do nothing and use the implicitly generated version, but when I wanted to move I obviously had to tell the compiler this:
e = Engine() calls Engine's move assignment operator. Since you don't have one, it's copy assignment operator will be called. Then RenderManger's copy assignment will be called. Finally it's the copy assignment of unique_ptr, which is deleted.
A move assignment is needed for Engine, as
Engine & operator = (Engine && rhs) { r._currentCamera = std::move(rhs.r._currentCamera); return * this; }
Suppose RenderManager::_currentCamera is visible to Engine, otherwise you need to defind move assignment for RenderManager.
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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().
0. In order to assign a new value to a std::unique_ptr use the reset() method. However, a big word of caution regarding the use of this method is that the std::unique_ptr object will try to dispose of the managed pointer by invoking a Deleter function on the managed pointer when the std::unique_ptr object will be being destroyed or when the ...
a stored deleter: a callable object that takes an argument of the same type as the stored pointer and is called to delete the managed object. It is set on construction, can be altered by an assignment operation, and can be individually accessed using member get_deleter. unique_ptr objects replicate a limited pointer functionality by providing ...
unique_ptr(const unique_ptr&)= delete; (7) 1) Constructs a std::unique_ptr that owns nothing. Value-initializes the stored pointer and the stored deleter. Requires that Deleter is DefaultConstructible and that construction does not throw an exception. These overloads participate in overload resolution only if std::is_default_constructible ...
Constructs a unique_ptr object, depending on the signature used: default constructor (1), and (2) The object is empty (owns nothing), with value-initialized stored pointer and stored deleter. construct from pointer (3) The object takes ownership of p, initializing its stored pointer to p and value-initializing its stored deleter. construct from pointer + lvalue deleter (4)
std::unique_ptr is by far the most used smart pointer class, so we'll cover that one first. In the following lessons, we'll cover std::shared_ptr and std::weak_ptr. std::unique_ptr. std::unique_ptr is the C++11 replacement for std::auto_ptr. It should be used to manage any dynamically allocated object that is not shared by multiple objects.
Return value (none) [] NoteTo replace the managed object while supplying a new deleter as well, move assignment operator may be used. A test for self-reset, i.e. whether ptr points to an object already managed by * this, is not performed, except where provided as a compiler extension or as a debugging assert.Note that code such as p. reset (p. release ()) does not involve self-reset, only code ...
The assignment operation between unique_ptr objects that point to different types (3) needs to be between types whose pointers are implicitly convertible, and shall not involve arrays in any case (the third signature is not part of the array specialization of unique_ptr). Copy assignment (4) to a unique_ptr type is not allowed (deleted signature).
In this article. A unique_ptr does not share its pointer. It cannot be copied to another unique_ptr, passed by value to a function, or used in any C++ Standard Library algorithm that requires copies to be made.A unique_ptr can only be moved. This means that the ownership of the memory resource is transferred to another unique_ptr and the original unique_ptr no longer owns it.
std::auto_ptr, and why to avoid it#. What we have seen above as smart_ptr is basically an std::auto_ptr which was introduced in C++98, was C++'s first attempt at a standardized smart pointer.; However, std::auto_ptr (and our smart_ptr class) has a number of problems that make using it dangerous. Because std::auto_ptr implements move semantics through the copy constructor and assignment ...
Description. 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 object is disposed of, using a potentially user-supplied deleter by calling ...
Copy assignment operator unique_ptr<T>& operator=(const unique_ptr<T>& uptr) = delete; This is redundant, this part is not necessary as this operator will never be called taking into account that the motion constructor exists and the copy constructor is disabled (= delete).
For the array specialization (unique_ptr<T[]>), this overload participates in overload resolution only if U is an array type, ... As a move-only type, unique_ptr's assignment operator only accepts rvalues arguments (e.g. the result of std::make_unique or a std::move 'd unique_ptr variable).
std::unique_ptr is a smart pointer introduced in C++11. It automatically manages the dynamically allocated resources on the heap. Smart pointers are just wrappers around regular old pointers that help you prevent widespread bugs. Namely, forgetting to delete a pointer and causing a memory leak or accidentally deleting a pointer twice or in the ...
A unique_ptr is a type of smart pointer provided by the C++ Standard Library that is designed to manage the memory of a dynamically allocated memory. It holds the exclusive ownership of the memory it points to, meaning there can be no other unique_ptr pointing to the same memory at the same time. This exclusive ownership is the first way in ...
The 2 statements are not equivalent. The second one value initializes the array, while the first one creates the array uninitialized. In that sense make_unique isn't always better than constructor plus new.That means for use-cases where value-initialization isn't needed and to be avoided because it is performance critical the constructor version is better.
In C++, a unique_ptr is a smart pointer that owns and manages dynamically allocated memory. It automatically deletes the linked memory when the unique_ptr goes out of scope. One key aspect of unique_ptr is ownership uniqueness, meaning that there can only be one unique_ptr pointing to any given resource at a time. This uniqueness is enforced by the language to prevent multiple unique_ptrs from ...
When working with unique_ptr in C++, it's essential to know how to renounce the ownership of the managed object and free the associated memory when necessary. The unique_ptr class provides the reset() member function for this purpose.. The unique_ptr::reset() Function. Calling the reset() method on a unique_ptr will perform two operations:. It will deallocate or release the memory that the ...
The usual case for one to have a unique_ptr in a class is to be able to use inheritance (otherwise a plain object would often do as well, see RAII). For this case, there is no appropriate answer in this thread up to now. So, here is the starting point: struct Base { //some stuff }; struct Derived : public Base { //some stuff }; struct Foo { std::unique_ptr<Base> ptr; //points to Derived or ...
e = Engine() calls Engine's move assignment operator. Since you don't have one, it's copy assignment operator will be called. Then RenderManger's copy assignment will be called. Finally it's the copy assignment of unique_ptr, which is deleted. A move assignment is needed for Engine, as. Engine & operator = (Engine && rhs) {.