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# std::allocate_shared

From cppreference.com
< cpp‎ | memory‎ | shared ptr

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`. 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`.

### [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

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.

### [edit] See also

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

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