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Showing content from https://cplusplus.com/reference/future/promise/promise/ below:

public member function

<future>

promise();

template <class Alloc> promise (allocator_arg_t aa, const Alloc& alloc);

promise (const promise&) = delete;

promise (promise&& x) noexcept;

Construct promise

(1) default constructor

The object is initialized with access to a new empty shared state.

(2) constructor with allocator

Same as (1), but uses alloc to allocate memory for the shared state.

(3) copy constructor [deleted]

promise objects cannot be copied.

(4) move constructor

The constructed object acquires the shared state of x (if any).
x is left with no shared state.

aa

The std::allocator_arg value. This constant value is merely used to explicitly select this constructor overload.

alloc

Allocator object.
The container keeps and uses an internal copy of this allocator, and uses it to allocate storage for the shared state.

x

Another promise object of the same type (with the same template parameter, T).

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// promise constructors
#include <iostream>       // std::cout
#include <functional>     // std::ref
#include <memory>         // std::allocator, std::allocator_arg
#include <thread>         // std::thread
#include <future>         // std::promise, std::future

void print_int (std::future<int>& fut) {
  int x = fut.get();
  std::cout << "value: " << x << '\n';
}

int main ()
{
  std::promise<int> foo;
  std::promise<int> bar = std::promise<int>(std::allocator_arg,std::allocator<int>());

  std::future<int> fut = bar.get_future();

  std::thread th (print_int, std::ref(fut));

  bar.set_value (20);

  th.join();
  return 0;
}

promise::operator=

Move-assign promise (public member function)

promise::set_value

Set value (public member function)

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