support bazel complie this project and format code.

learn_class/modern_cpp_30/reference/reference.cpp

@@ -0,0 +1,43 @@
+//
+// Created by light on 19-12-14.
+//
+
+#include <iostream>
+#include <functional>
+
+using namespace std;
+
+// xvalue
+int&& f(){
+    return 3;
+}
+
+struct As
+{
+    int i;
+};
+
+As&& ff(){
+    return As();
+}
+
+int main() {
+    // lvalue
+    int x = 0;
+    cout << "(x).addr = " << &x << endl;
+    cout << "(x = 1).addr = " << &(x = 1) << endl;
+    cout << "(++x).addr = " << &++x << endl;
+    cout << "(cout << ' ').addr=" << &(cout << ' ') << endl;
+    cout << "(\"hello\").addr=" << &("hello") << endl;
+    // rvalue
+    cout<<true<<endl;
+    // xvalue
+    f(); // The expression f() belongs to the xvalue category, because f() return type is an rvalue reference to object type.
+    static_cast<int&&>(7); // The expression static_cast<int&&>(7) belongs to the xvalue category, because it is a cast to an rvalue reference to object type.
+    std::move(7); // std::move(7) is equivalent to static_cast<int&&>(7).
+
+    ff().i; // The expression f().i belongs to the xvalue category, because As::i is a non-static data member of non-reference type, and the subexpression f() belongs to the xvlaue category.
+
+
+    return 0;
+}