play with std::future

.gitignore

@@ -18,3 +18,4 @@ classes
 lvalues
 bad_alloc
 rtti
+future

CMakeLists.txt

@@ -18,3 +18,6 @@ add_executable(classes classes.cpp)
 add_executable(lvalues lvalues.cpp)
 add_executable(bad_alloc bad_alloc.cpp)
 add_executable(rtti rtti.cpp)
+
+add_executable(future future.cpp)
+set_target_properties(future PROPERTIES LINK_FLAGS "-pthread")

future.cpp

@@ -0,0 +1,47 @@
+// future example
+// adapted from http://www.cplusplus.com/reference/future/future/
+
+#include <iostream>       // std::cout
+#include <future>         // std::async, std::future
+#include <chrono>         // std::chrono::milliseconds
+
+// a non-optimized way of checking for prime numbers.
+bool is_prime (int x) {
+  for (int i=2; i<x; ++i) {
+    if (x%i == 0) return false;
+  }
+  return true;
+}
+
+void explicitly_waiting() {
+  int p = 817504243;
+
+  // call function asynchronously:
+  std::future<bool> fut = std::async(std::launch::async, is_prime, p);
+
+  // do something while waiting for function to set future:
+  std::cout << "checking, please wait" << std::flush;
+  std::chrono::milliseconds span(500);
+  while (fut.wait_for(span) == std::future_status::timeout) {
+    std::cout << '.' << std::flush;
+  }
+  std::cout << std::endl;
+
+  bool x = fut.get();     // retrieve return value
+  std::cout << p << " " << (x?"is":"is not") << " prime." << std::endl;
+}
+
+void implicitly_waiting() {
+  int p2 = 838041641;
+
+  std::future<bool> fut = std::async(std::launch::async, is_prime, p2);
+  std::cout << "just getting the result, doing an implicit wait():" << std::endl;
+  bool x = fut.get();
+  std::cout << p2 << " " << (x?"is":"is not") << " prime." << std::endl;
+}
+
+int main () {
+  explicitly_waiting();
+  implicitly_waiting();
+  return 0;
+}