std::allocate_shared

template< class T, class Alloc, class... Args >
shared_ptr<T> allocate_shared( const Alloc& alloc, Args&&... args );

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. The object is constructed as if by the expression ::new (pv) T(std::forward<Args>(args)...), where pv is an internal void* pointer to storage suitable to hold an object of type T. The storage is typically larger than sizeof(T) in order to use one allocation for both the control block of the shared pointer and the T object.

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.

Parameters

Return value

std::shared_ptr of an instance of type T.

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.

Notes

Like std::make_shared, this function typically performs only one allocation, and places both the T object and the control block in the allocated memory block (the standard recommends, but does not require this, all known implementations do this). 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.

Unlike the std::shared_ptr constructors, std::allocate_shared does not accept a separate custom deleter: the supplied allocator is used for destruction of the control block and the T object, and for deallocation of their shared memory block.

std::shared_ptr supports array types (as of C++17), but std::allocate_shared does not. This functionality is supported by boost::allocate_shared.

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,
                                  const_cast<std::remove_cv_t<U>*>(ptr));

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.

See also