assignment to auto_ptr

Hi,
I am learning to use auto_ptr.
my question is in purple comments below.

Code:

---


#include <iostream>
using namespace std;

class Base
{
public:
    virtual void print()=0;
};

class A : public Base
{
public:
    void print() { cout << "A" << endl; }
};

class B : public Base
{
public:
    void print() { cout << "B" << endl; }
};

int main()
{
    auto_ptr<Base> owner;
   
    if(5==5)
    {
        A* a = new A;
        owner = a;    // this was my first guess, does not compile though.
        owner = auto_ptr<Base>(a); // this compiles. is this how you do it?
    }
    else
    {
        B* b = new B;
        owner = auto_ptr<Base>(b);
    }

    owner->print();

    return 0;
}

---

Yes, the second way is correct. You could just write:

Code:

---

owner = auto_ptr<Base>(new A);

---

Incidentally, I believe you should #include <memory> for std::auto_ptr.

Thanks laserlight.

You should do

Code:

---

owner.reset(new A);

---

Quote:

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Originally Posted by CornedBee

You should do

Code:

---

owner.reset(new A);

---

---

Why? isn't it the same?

No, it avoids constructing a new auto_ptr and then copy-assigning it into the current one. Now, compilers might be able to optimize the additional copy away - but then, they might not.

In any case, the reset() is the most common way of setting a smart pointer to a new value.

>Why?
It's shorter and more directly reflects your intentions?

Quote:

---

Originally Posted by kroiz

Why? isn't it the same?

---

not exactly

Code:

---

owner = auto_ptr<Base>(new A);

---

creates an unnecessary temporary auto_ptr that is then assigned to owner.
Kurt

Understood.
Thanks you all.

Is it the same with boost::shared_ptr regarding .reset() thing?

Yes. reset() allows smart pointers to take ownership of an object by swapping it into its this pointer. That's all reset does, and you should use it.