< cpp‎ | memory‎ | shared ptr
Dynamic memory management
Uninitialized storage
(deprecated since c++17)
(deprecated since c++17)
(deprecated since c++17)
Garbage collection support
C Library
Low level memory management
Defined in header <memory>
template< class T, class Alloc, class... Args >
shared_ptr<T> allocate_shared( const Alloc& alloc, Args&&... args );
(since C++11)

Constructs an object of type T and wraps it in a std::shared_ptr using args as the parameter list for the constructor of T.

All memory allocation is done using a copy of alloc, which must satisfy the Allocator requirements.

The std::shared_ptr constructor called by this function enables shared_from_this with a pointer to the newly constructed object of type T.


[edit] Parameters

alloc - The Allocator to use.
args... - list of arguments with which an instance of T will be constructed.

[edit] Return value

std::shared_ptr of an instance of type T.

[edit] Exceptions

Can throw the exceptions thrown from Alloc::allocate() or from the constructor of T. If an exception is thrown, this function has no effect.

[edit] Notes

This function typically allocates memory for the T object and for the shared_ptr's control block with a single memory allocation (it is a non-binding requirement in the Standard). In contrast, the declaration std::shared_ptr<T> p(new T(Args...)) performs at least two memory allocations, which may incur unnecessary overhead.

A copy of alloc is stored as part of the control block so that it can be used to deallocate it once both shared and weak reference counts reach zero.

A constructor enables shared_from_this with a pointer ptr of type U* means that it determines if U has an unambiguous and accessible base class that is a specialization of std::enable_shared_from_this, and if so, the constructor evaluates the statement:

if (ptr != nullptr && ptr->weak_this.expired())
  ptr->weak_this = std::shared_ptr<std::remove_cv_t<U>>(*this,

Where weak_this is the hidden mutable std::weak_ptr member of std::shared_from_this. The assignment to the weak_this member is not atomic and conflicts with any potentially concurrent access to the same object. This ensures that future calls to shared_from_this() would share ownership with the shared_ptr created by this raw pointer constructor.

The test ptr->weak_this.expired() in the exposition code above makes sure that weak_this is not reassigned if it already indicates an owner. This test is required as of C++17.

[edit] See also

constructs new shared_ptr
(public member function) [edit]
creates a shared pointer that manages a new object
(function template) [edit]