lst-0046-book.cpp
// https://godbolt.org/z/7Mea1Y3Mn
#include <iostream>
#include <future> // async
using std::cout; using std::endl;
long fib(long n) { return n<=1 ? n : fib(n-1)+fib(n-2); }
int main() {
auto f40 = std::async(fib, 40);
auto f41 = std::async(fib, 41);
auto f42 = std::async(fib, 42);
auto f43 = std::async(fib, 43);
cout << "fib(40): " << f40.get() << endl; // Output: fib(40): 102334155
} // also waits for f41, f42, and f43.