c++ - Double move on same object is copying from left to right? -

i beginner in move operation in c++11, playing it. found not able understand.

#include <iostream> using namespace std;  class a{     public:         a(){cout << "default ctor" << endl;}         a(const string& str):_str{str}{cout << "parameter ctor" << endl;}         a(a&& obj):_str{std::move(obj._str)}{cout << "move ctor" << endl;}         a& operator =(a&& rhs){_str = std::move(rhs._str);cout << "move assignment operation" << endl; return *this;}         void print(){cout << _str << endl;}     private:         string _str; };  int main(){     a("rupesh yadav"); // parameter ctor     b(std::move(a));   // move ctor      cout << "print a: ";     a.print();           // not printing  --> correct!!     cout << "print b: ";     b.print();           // printing      --> correct!!      b = std::move(a);    // don't know may silly still lets why not!!!, mistake??      cout << "print a: ";      a.print();           // printing      --> wrong!!      cout << "print b: ";      b.print();           // not printing  --> wrong!! } 

i expecting b = std::move(a) operation behave different because applying move on object a second time copying left side object b right hand side object a, part don't understand.

or have done wrong in programming. please if doing wrong in move operation.

edit: know undefined behavior. doubt if again copying object a object b, , if again same thing copy object b object a?

hence copying form left right , right left why?

you can't move same object twice.

after first moved a b, a had "valid unspecified state" (can't remember exact terminology). then, attempted move a b again! heck has broken loose. (i suspect that, internally, data pointers have been swapped around.)

simply don't this. see no reason want to.