add: exercise answer for chapter 6 and 7

exercises/7/main.cpp

@@ -0,0 +1,40 @@
+#include <iostream> // std::cout, std::endl
+
+#include <vector>   // std::vector
+#include <string>   // std::string
+#include <future>   // std::future
+#include <thread>   // std::this_thread::sleep_for
+#include <chrono>   // std::chrono::seconds
+
+#include "ThreadPool.hpp"
+
+int main()
+{
+    // create a thread pool with max. 4 concurrency threads
+    ThreadPool pool(4);
+    // create execution results list
+    std::vector< std::future<std::string> > results;
+
+    // start eight thread task
+    for(int i = 0; i < 8; ++i) {
+        // add all task to result list
+        results.emplace_back(
+            // ass print task to thread pool
+            pool.enqueue([i] {
+                std::cout << "hello " << i << std::endl;
+                // wait a sec when the previous line is out
+                std::this_thread::sleep_for(std::chrono::seconds(1));
+                // keep output and return the status of execution
+                std::cout << "world " << i << std::endl;
+                return std::string("---thread ") + std::to_string(i) + std::string(" finished.---");
+            })
+        );
+    }
+
+    // outputs
+    for(auto && result: results)
+        std::cout << result.get() << ' ';
+    std::cout << std::endl;
+    
+    return 0;
+}