book: translate ch03

see #12

code/3/3.6.move.semantics.cpp

@@ -0,0 +1,31 @@
+//
+// 3.5.move.semantics.cpp
+// modern c++ tutorial
+//
+// created by changkun at changkun.de
+// https://github.com/changkun/modern-cpp-tutorial
+//
+
+#include <iostream> // std::cout
+#include <utility>  // std::move
+#include <vector>   // std::vector
+#include <string>   // std::string
+
+int main() {
+    
+    std::string str = "Hello world.";
+    std::vector<std::string> v;
+    
+    // use push_back(const T&), copy
+    v.push_back(str);
+    // "str: Hello world."
+    std::cout << "str: " << str << std::endl;
+    
+    // use push_back(const T&&), no copy
+    // the string will be moved to vector, and therefore std::move can reduce copy cost
+    v.push_back(std::move(str));
+    // str is empty now
+    std::cout << "str: " << str << std::endl;
+    
+    return 0;
+}