book: translate ch03

see #12

code/3/3.3.rvalue.cpp

@@ -0,0 +1,38 @@
+//
+// 3.3.rvalue.cpp
+// modern c++ tutorial
+//
+// created by changkun at changkun.de
+// https://github.com/changkun/modern-cpp-tutorial
+//
+
+
+#include <iostream>
+#include <string>
+
+void reference(std::string& str) {
+    std::cout << "lvalue" << std::endl;
+}
+void reference(std::string&& str) {
+    std::cout << "rvalue" << std::endl;
+}
+
+int main()
+{
+    std::string  lv1 = "string,";       // lv1 is a lvalue
+    // std::string&& r1 = s1;           // illegal, rvalue can't ref to lvalue
+    std::string&& rv1 = std::move(lv1); // legal, std::move can convert lvalue to rvalue
+    std::cout << rv1 << std::endl;      // string,
+    
+    const std::string& lv2 = lv1 + lv1; // legal, const lvalue reference can extend temp variable's lifecycle
+    // lv2 += "Test";                   // illegal, const ref can't be modified
+    std::cout << lv2 << std::endl;      // string,string
+    
+    std::string&& rv2 = lv1 + lv2;      // legal, rvalue ref extend lifecycle
+    rv2 += "string";                    // legal, non-const reference can be modified
+    std::cout << rv2 << std::endl;      // string,string,string,
+    
+    reference(rv2);                     // output: lvalue
+    
+    return 0;
+}