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The C++ Standard says that an STL element must be "copy-constructible" and "assignable." These fancy words simply mean you can safely assign or copy one object to another and get two independent and logically identical copies. In particular, the state of the original object shouldn't change when it is copied to the target object.

This is not the case with auto_ptr, though: copying or assigning one auto_ptr to another makes changes to the original, in addition to the obvious changes to the copy. To be more specific, the original object transfers ownership of the pointer to the target, and the pointer in the original object becomes null. Imagine what would happen if you did something like this:


class Foo{};
vector < auto_ptr <Foo> > vf; //a vector of auto_ptr's
//..fill vf
int g()
{
auto_ptr <Foo> temp = vf[0]; // vf[0] becomes null
}


When temp is initialized, the member vf[0] is changed: its pointer becomes null. Any attempt to use that element will cause a runtime crash. This situation is likely to occur whenever you copy an element from the container. Remember that even if your code does not perform any explicit copy or assignment operations, many algorithms (swap(), random_shuffle(), sort(), and many others) create a temporary copy of one or more container elements. Furthermore, certain member functions of the container may create a temporary copy of one or more elements, thereby nullifying them. Any subsequent attempt to the container elements is therefore undefined.

auto_ptr does not copy well. For this reason, putting them in containers like vectors, queues is not recommended.