std::auto_ptr becomes invalid after being passed to a function by value

Question

I've got the following sample code:

#include <iostream>
#include <auto_ptr.h>

class A
{
public:
    A(){ std::cout << "A ctor" << std::endl;}
    ~A() {std::cout << "A dtor" << std::endl;}
    void bar(){std::cout << "bar()" << std::endl;}
};

void foo(std::auto_ptr<A> a)
{
    std::cout << "foo()" << std::endl ;
}

int main()
{
    std::auto_ptr<A> a(new A());
    a->bar();
    return 0;
}

output:

A ctor
bar()
A dtor

Now if I call foo(a), a will be destructed before calling bar():

int main()
{
    std::auto_ptr<A> a(new A());
    foo(a);
    a->bar();
    return 0;
}

output:

A ctor
foo()
A dtor
bar()

Why is a destructed after foo() is called?

Another thing I don't understand is that if I pass the parameter to foo by reference, a will not be destructed after calling foo():

void foo(std::auto_ptr<A> &a)
{
    std::cout << "foo()" << std::endl ;
}

int main()
{
    std::auto_ptr<A> a(new A());
    foo(a);
    a->bar();
    return 0;
}

output:

A ctor
foo()
bar()
A dtor

How is passing by reference affecting the lifetime of auto_ptr?

Answer 1

auto_ptr steals ownership upon copying. When you copy it, the copy now holds the pointer to the object, and the original holds nothing.

It's also a deprecated concept. If you have access to C++11, use unique_ptr instead.