see #2: update std::atomic and memory model

code/7/7.6.atomic.cpp

@@ -0,0 +1,19 @@
+#include <atomic>
+#include <thread>
+#include <iostream>
+
+std::atomic<int> count = {0};
+
+int main() {
+    std::thread t1([](){
+        count.fetch_add(1);
+    });
+    std::thread t2([](){
+        count++;        // identical to fetch_add
+        count += 1;     // identical to fetch_add
+    });
+    t1.join();
+    t2.join();
+    std::cout << count << std::endl;
+    return 0;
+}

code/7/7.7.is.lock.free.cpp

@@ -0,0 +1,12 @@
+#include <atomic>
+#include <iostream>
+
+struct A {
+    float x;
+    int y;
+    long long z;
+};
+    std::atomic<A> a;
+    std::cout << std::boolalpha << a.is_lock_free() << std::endl;
+    return 0;
+}

code/7/7.8.relaxed.cpp

@@ -0,0 +1,21 @@
+#include <atomic>
+#include <thread>
+#include <vector>
+#include <iostream>
+
+std::atomic<int> counter = {0};
+
+int main() {
+    std::vector<std::thread> vt;
+    for (int i = 0; i < 100; ++i) {
+        vt.emplace_back([](){
+            counter.fetch_add(1, std::memory_order_relaxed);
+        });
+    }
+
+    for (auto& t : vt) {
+        t.join();
+    }
+    std::cout << "current counter:" << counter << std::endl;
+    return 0;
+}