update

design_pattern/singleton/barrier_singleton.cpp

@@ -0,0 +1,46 @@
+//
+// Created by light on 20-2-7.
+//
+#include <iostream>
+
+using namespace std;
+
+#include <mutex>
+
+#define barrier() __asm__ volatile ("lwsync")
+
+// method 1 operator new + placement new
+//singleton *instance() {
+//    if (p == nullptr) {
+//        lock_guard<mutex> guard(lock_);
+//        if (p == nullptr) {
+//            singleton *tmp = static_cast<singleton *>(operator new(sizeof(singleton)));
+//            new(p)singleton();
+//            p = tmp;
+//        }
+//    }
+//    return p;
+//}
+class singleton {
+private:
+    singleton() {}
+
+    static singleton *p;
+    static mutex lock_;
+public:
+    static singleton *instance();
+};
+
+singleton *singleton::p = nullptr;
+
+singleton *singleton::instance() {
+    if (p == nullptr) {
+        lock_guard<mutex> guard(lock_);
+        barrier();
+        if (p == nullptr) {
+            p = new singleton();
+        }
+    }
+    return p;
+}
+