Smart Pointer (auto_ptr) behavior

Question

Not sure if someone has already asked this, but I see a strange behavior here:

I've declared two classes, one base and one derived with just one virtual method display().

class A {
public:
    virtual void display() {
        cout << "base class" << endl;
    }
};

class B:public A {
public:
    void display() {
        cout << "derived class" << endl;
    }
};

Now, in main(), if I try to declare an auto_ptr of A and assign it a new instance of B,

int main() {
    auto_ptr<A> *a = (auto_ptr<A>*)new B();
    a->display();
}

I get this error on compiling:

"'class std::auto_ptr<A>' has no member named 'display'"

Am I doing something wrong? Can someone explain the behavior?

Answer 1

You are creating a pointer to an auto_ptr. An auto_ptr is an object that works like a pointer, so you don't need to add a *.

You probably want:

auto_ptr<A> a(new B());
a->display();

Although I must recomment either Boost's smart pointers (scoped_ptr and shared_ptr) or C++11's std::unique_ptr and std::shared_ptr.

Answer 2

auto_ptr<A> *a = (auto_ptr<A>*)new B();

That is doing something very strange. If you want to create an object and use a smart pointer to manage it, then initialise the smart pointer with a pointer to the object:

auto_ptr<A> a(new B);
a->display();

Answer 3

Why did you write auto_ptr<A> *a? It should not be like that. Therefore you are getting this error. It should be auto_ptr<A> a(new B);. Read here how it works.