23 General utilities library [utilities]

23.11 Smart pointers [smartptr]

23.11.2 Shared-ownership pointers [util.smartptr]

23.11.2.2 Class template shared_ptr [util.smartptr.shared]

23.11.2.2.6 shared_ptr creation [util.smartptr.shared.create]

template<class T, class... Args>
  shared_ptr<T> make_shared(Args&&... args);
template<class T, class A, class... Args>
  shared_ptr<T> allocate_shared(const A& a, Args&&... args);

Requires: The expression ::new (pv) T(std::forward<Args>(args)...), where pv has type void* and points to storage suitable to hold an object of type T, shall be well formed.

A shall be an allocator ([allocator.requirements]).

The copy constructor and destructor of A shall not throw exceptions.

Effects: Allocates memory suitable for an object of type T and constructs an object in that memory via the placement new-expression ::new (pv) T(std::forward<Args>(args)...).

The template allocate_shared uses a copy of a to allocate memory.

If an exception is thrown, the functions have no effect.

Returns: A shared_ptr instance that stores and owns the address of the newly constructed object of type T.

Postconditions: get() != 0 && use_count() == 1.

Throws: bad_alloc, or an exception thrown from A::allocate or from the constructor of T.

Remarks: The shared_ptr constructor called by this function enables shared_from_this with the address of the newly constructed object of type T.

Implementations should perform no more than one memory allocation.

[ Note

This provides efficiency equivalent to an intrusive smart pointer.

— end note

]