std::future valid()

The std::future valid() function is used to check if a std::future object is valid, i.e., if it is associated with any shared state. The function returns true if the std::future object is valid and false otherwise.

Here's an example:

#include <iostream>
#include <future>

int main() {
  std::future<int> fut;
  std::cout << "Is future valid? " << fut.valid() << std::endl;
  
  fut = std::async(std::launch::async, [](){ return 42; });
  std::cout << "Is future valid? " << fut.valid() << std::endl;
  
  int result = fut.get();
  std::cout << "Result: " << result << std::endl;
  std::cout << "Is future valid? " << fut.valid() << std::endl;
  
  return 0;
}

Output:

Is future valid? 0
Is future valid? 1
Result: 42
Is future valid? 0

In this example, we create a std::future object `fut` and check if it is valid using the `valid()` function, which returns false as `fut` is not associated with any shared state.

Next, we create a task using `std::async()` and assign the returned future object to `fut`. We then check if `fut` is valid using the `valid()` function, which returns true as `fut` is now associated with a shared state.

We then call `fut.get()` to retrieve the result of the task and store it in `result`. After that, we check if `fut` is still valid using the `valid()` function, which now returns false as the shared state has been consumed.