#include <future>
#include <chrono>
#include <iostream>

int main()
{
    auto show = [](auto&& t) { std::cout << t << std::endl; };

    // Let's create a simple "future" object.
    std::future<int> value;

    // There was nothing that supplies a value to the
    // "future" object. For instance, these will not compile:
    //value = 4;    // Doesn't compile!
    //value.set(4); // Doesn't compile!

    // Right now, it contains no value.

    // Is it valid?
    show( value.valid() ); // false

    // Let's wait until it contains a value!
    // But only wait 2 seconds at most.
    value.wait_for(std::chrono::seconds(2));
    // value.wait(); // Wait indefinitely

    // What is the value now? Note: get() also calls wait().
    show( value.get() ); // std::future_error "No associated state"

    // The value can only be read once. Calling get() again
    // would produce std::future_error "No associated state".
}