add Chapter06

Chapter06/lvalue_refs.cpp

@@ -0,0 +1,26 @@
+#include <cstdio> 
+
+  
+
+int main() { 
+
+    int a = 42;  
+
+    int& a_ref = a; 
+
+    const int& a_const_ref = a; 
+
+
+    printf("a = %d\r\n", a); 
+
+  
+    a_ref = 16; 
+
+    printf("a = %d\r\n", a); 
+
+
+    // a_const_ref = 16; compiler error 
+
+    return 0;  
+
+}