Re: Object that transfers ownership on assignment/copy

"Victor Bazarov" <>
Wed, 5 Sep 2007 17:04:25 -0400
Scott Gifford wrote:

As a possible solution to a problem I'm trying to solve with an
iterator (see an earlier post by me with subject "Iterator
implementation questions: copy constructor and postfix increment"),
I'm trying to implement an object that transfers ownership on
assignment or copy, just like auto_ptr does.

With an auto_ptr, I can do this:

   #include <string>
   #include <memory>
   #include <iostream>

   using namespace std;

   auto_ptr<string> ap_factory() {
     return auto_ptr<string>(new string("foo"));

   int main() {
     auto_ptr<string> s = ap_factory();
     auto_ptr<string> s2 = s;
     cout << "s1=" << s.get() << ", s2=" << *s2 << endl;

and when "s2 = s" executes, the pointer that was held by "s" is
trasferred to "s2", and then the pointer in "s" is set to NULL. So, I
see this output:

   s1=0, s2=foo

When I try to implement the same behavior in my own class, I have
compilation problems. Here's my small test case:

   #include <string>
   #include <iostream>
   using namespace std;

   class C {

       C(string *_s) : s(_s) { }

       C(C &that) : s(that.s) { }

       C& operator=(C &that) {
         if (&that != this) {
           s = that.s;
         return *this;

       const string *get() {
         return s.get();

       auto_ptr<string> s;

   C c_factory() {
     return C(new string("foo"));

   int main() {
     C myC = c_factory();

Here you have a temporary from which you're trying to construct
an object. That operation requires binding a non-const reference
to the temporary, which is prohibited.

Take a look at how 'auto_ptr' is implemented and follow that instead
of trying to wrap it up.

     C myC2 = myC;
     cout << "myC=" << myC.get() << ", myC2 = " << *(myC2.get()) <<

     return 0;


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