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std::future::get(3)	      C++ Standard Libary	   std::future::get(3)

NAME
       std::future::get	- std::future::get

Synopsis
	  T get();    (1) (member only of generic future template)
			  (since C++11)
	  T& get();   (2) (member only of future<T&> template specialization)
			  (since C++11)
	  void	get();	(3)  (member only of future<void> template specializa-
       tion)
			  (since C++11)

	  The get member function waits	until the future has  a	 valid	result
       and (depending on
	  which	template is used) retrieves it.	It effectively calls wait() in
       order to	wait
	  for the result.

	  The generic template and two template	specializations	each contain a
       single version
	  of get. The three versions of	get differ only	in the return type.

	  The  behavior	 is  undefined	if valid() is false before the call to
       this function.

	  Any shared state is released.	valid()	is false after a call to  this
       member function.

Parameters
	  (none)

Return value
	  1) The value v stored	in the shared state, as	std::move(v).
	  2) The reference stored as value in the shared state.
	  3) Nothing.

Exceptions
	  If an	exception was stored in	the shared state referenced by the fu-
       ture (e.g. via a
	  call	to  std::promise::set_exception()) then	that exception will be
       thrown.

Notes
	  The implementations are encouraged to	detect the case	 when  valid()
       is false	before
	  the call and throw a std::future_error with an error condition of
	  std::future_errc::no_state.

Example
       // Run this code

	#include <thread>
	#include <future>
	#include <iostream>
	#include <string>
	#include <chrono>

	std::string time() {
	    static auto	start =	std::chrono::steady_clock::now();
	    std::chrono::duration<double> d = std::chrono::steady_clock::now()
       - start;
	    return "[" + std::to_string(d.count()) + "s]";
	}
	int main() {
	    using namespace std::chrono_literals;
	    {
		std::cout << time() << " launching thread\n";
		std::future<int> f = std::async(std::launch::async, []{
		    std::this_thread::sleep_for(1s);
		    return 7;
		});
		std::cout  << time() <<	" waiting for the future, f.valid() ==
       "
			  << f.valid() << "\n";
		int n =	f.get();
		std::cout << time() << " future.get() returned with " << n  <<
       ". f.valid() = "
			  << f.valid() << '\n';
	    }

	    {
		std::cout << time() << " launching thread\n";
		std::future<int> f = std::async(std::launch::async, []{
		    std::this_thread::sleep_for(1s);
		    return true	? throw	std::runtime_error("7")	: 7;
		});
		std::cout  << time() <<	" waiting for the future, f.valid() ==
       "
			  << f.valid() << "\n";
		try {
		    int	n = f.get();
		    std::cout << time()	<< " future.get() returned with	" << n
			      << " f.valid() = " << f.valid() << '\n';
		} catch(const std::exception& e) {
		    std::cout << time()	<< " caught exception "	<< e.what()
			      << ", f.valid() == " << f.valid()	<< "\n";
		}
	    }
	}

Possible output:
	[0.000004s] launching thread
	[0.000461s] waiting for	the future, f.valid() == 1
	[1.001156s] future.get() returned with 7. f.valid() = 0
	[1.001192s] launching thread
	[1.001275s] waiting for	the future, f.valid() == 1
	[2.002356s] caught exception 7,	f.valid() == 0

See also
	  valid	checks if the future has a shared state
		(public	member function)

http://cppreference.com		  2022.07.31		   std::future::get(3)

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