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?

I’ve fixed your formatting. You can use the preview when writing the question to see whether the format is correct; in your case, you mixed up quoted text with source code. — Konrad Rudolph, May 15 '12 at 09:35
@KonradRudolph Thanks. I'll take care of this next time. I'm back at StackOverflow and to C++ programming after a long time and hence these errors :). — Shree, May 15 '12 at 10:11

score 3 · Accepted Answer · answered May 15 '12 at 09:32

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.

score 2 · Answer 2 · answered May 15 '12 at 09:32

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();

answered May 15 '12 at 09:32

Mike Seymour

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score 2 · Answer 3 · answered May 15 '12 at 09:33

2

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.

answered May 15 '12 at 09:33

besworland

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Smart Pointer (auto_ptr) behavior

3 Answers3