std::allocate_shared, std::allocate_shared_default_init

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

template< class T, class Alloc >
shared_ptr<T> allocate_shared( const Alloc& alloc, std::size_t N );

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

template< class T, class Alloc >
shared_ptr<T> allocate_shared( const Alloc& alloc, std::size_t N,
                               const std::remove_extent_t<T>& u );

template< class T, class Alloc >
shared_ptr<T> allocate_shared( const Alloc& alloc,
                               const std::remove_extent_t<T>& u );

template< class T, class Alloc >
shared_ptr<T> allocate_shared_default_init( const Alloc& alloc );

template< class T, class Alloc >
shared_ptr<T> allocate_shared_default_init( const Alloc& alloc, std::size_t N );

For allocate_shared, the object (or the individual array elements for (2-5)) (since C++20) are destroyed via the expression std::allocator_traits<A2>::destroy(a, p), where p is a pointer to the object and a is a copy of the allocator passed to allocate_shared, rebound to the type of the object being destroyed.

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, (1) has no effect. If an exception is thrown during the construction of the array, already-initialized elements are destroyed in reverse order (since C++20).

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.

A constructor enables shared_from_this with a pointer ptr of type U* means that it determines if U has an unambiguous and accessible (since C++17) 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