# std::experimental::promise<R>::promise (library fundamentals TS)

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< cpp‎ | experimental‎ | lib extensions‎ | promise

promise(); | (1) | (library fundamentals TS)
template< class Alloc > promise( std::allocator_arg_t, const Alloc& alloc ); | (2) | (library fundamentals TS)
promise( promise&& other ) noexcept; | (3) | (library fundamentals TS)
promise( const promise& other ) = delete; | (4) | (library fundamentals TS)

Constructs a `std::experimental::promise` object.

1) Default constructor. Constructs the promise with an empty shared state.
2) Constructs the promise with an empty shared state. The shared state is allocated using `alloc`, which is treated as a type-erased allocator (see below).
3) Move constructor. Constructs the promise with the shared state of `other` using move semantics. After construction, `other` has no shared state.
4) `std::experimental::promise` is not copyable.

### Type-erased allocator

The constructors of `promise` taking an allocator argument `alloc` treats that argument as a type-erased allocator. The memory resource pointer used by `promise` to allocate memory is determined using the allocator argument (if specified) as follows:

Type of `alloc` | value of the memory resource pointer
Non-existent (no allocator specified at time of construction) | The value of std::experimental::pmr::get_default_resource () at time of construction.
std::nullptr_t | The value of std::experimental::pmr::get_default_resource () at time of construction
A pointer type convertible to std::experimental::pmr::memory_resource * | static_cast< std::experimental::pmr::memory_resource *>(alloc)
A specialization of std::experimental::pmr::polymorphic_allocator | alloc.resource()
Any other type meeting the Allocator requirements | A pointer to a value of type std::experimental::pmr::resource_adaptor <A>(alloc), where `A` is the type of `alloc`. The pointer remains valid only for the lifetime of the `promise` object.
None of the above | The program is ill-formed.

### Parameters

alloc | - | allocator to use to allocate the shared state
other | - | another `std::experimental::promise` to acquire the state from

### Exceptions

1-2) (none)
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