LinuxQuestions.org - [SOLVED] Destructor called on objects in deque without it being called explicitly

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Destructor called on objects in deque without it being called explicitly

I feel there's a bit of a misunderstanding going on between C++ and me...

My problem is that I have a deque of pointers to a class, and create objects then add them to the deque in a for loop. However, the destructor is called on the object as soon as the loop quits.

Code:

#include <deque>

#include "stdio.h"



class Test {

    private:

        static int number;

        int id;

    public:

        Test(void);

        Test(const Test&);

        ~Test(void);

};

 

int Test::number = 0;



Test::Test(void){

    id = ++number;

    printf("Creating number %d\n",id);

}



Test::Test(const Test& old){

    id = old.id;

    printf("Copying number %d\n",id);

}



Test::~Test(void){

    printf("Deleting number %d\n",id);

}



int main(void){

    std::deque<Test*> myDeque;

    int i;

    for(i=0;i<5;i++){

        printf("\ni is now %d\n",i);

        Test temp;

        myDeque.push_front(&temp);

    }

    printf("Out of loop\n\n");

    return 0;

}

This prints:

Code:

i is now 0

Creating number 1

Deleting number 1



i is now 1

Creating number 2

Deleting number 2



i is now 2

Creating number 3

Deleting number 3



i is now 3

Creating number 4

Deleting number 4



i is now 4

Creating number 5

Deleting number 5

Out of loop

Now, if I change the deque from deque<Test*> to deque<Test> then it works fine - I get:

Code:

i is now 1

Creating number 1

Copying number 1

Deleting number 1

...

Out of loop

...

Deleting number 5

Deleting number 4

...

Deleting number 1

However, I can't do this with my actual code, because I am creating a deque of pointers to an abstract class, so it won't let me instantiate the class. (This is all so that I can take advantage of polymorphism, and call functions on a set of similar objects without worrying what type of object I'm dealing with).

So, I suppose my question is: what is the correct way to create that deque of pointers to the abstract class? I don't seem to be barking up the right tree currently.

Thanks in advance :)

You've declared a local object in your for-loop, which gets destroyed with each iteration of the loop.

Code:

Test temp;

If you want the objects to persist beyond the scope of the loop, then do something like:

Code:

Test* temp = new Test;

myDeque.push_front(temp);

P.S. Don't forget to delete the allocated object(s) at a later time.

Brilliant, thank you very much.

With regards to your postscript, do you just mean that when popping the objects I should do:

Code:

delete mydeque.back();

mydeque.pop_back();

Or do I need to do something more to ensure they are deleted correctly?

Quote:

Originally Posted by Snark1994 (Post 4413673)

Brilliant, thank you very much.

With regards to your postscript, do you just mean that when popping the objects I should do:

Code:

delete mydeque.back();

mydeque.pop_back();

Or do I need to do something more to ensure they are deleted correctly?

What you have above is fine; however don't you want to have a handle to the Test object to do something with it?

Code:

Test* test = mydeque.back();

mydeque.pop_back();



// do something with test

...



delete test;

Yes, sorry, I just meant for how I go about deleting the object. Thankyou :)

However, it turns out there's an additional complication due to the fact that I'm using an abstract class.

Code:

#include <deque>

#include "stdio.h"



class MetaTest {

        public:

                virtual void something(void)=0;

};



class Test: public MetaTest {

        private:

                static int number;

                int id;

        public:

                Test(void);

                Test(const Test&);

                ~Test(void);

                void something(void){};

};



int Test::number = 0;





Test::Test(void){

        id = ++number;

        printf("Creating number %d\n",id);

}



Test::Test(const Test& old){

        id = old.id;

        printf("Copying number %d\n",id);

}



Test::~Test(void){

        printf("Deleting number %d\n",id);

}



int main(void){

        std::deque<MetaTest*> myDeque;

        int i;

        for(i=0;i<5;i++){

                printf("\ni is now %d\n",i);

                Test* temp = new Test;

                myDeque.push_front(temp);

        }

        printf("Out of loop\n\n");



        printf("Starting to delete:\n");

        while(myDeque.size() > 0){

                delete myDeque.back();

                myDeque.pop_back();

        }

        return 0;

}

My guess is that because I'm using a pointer to the MetaTest class, it doesn't know it has to call the Test destructor. Is there a way around this? Or do I have to write another virtual method in the MetaTest called "deleteMe()" which is then overridden in each of the derived classes and called before I delete the object?

EDIT: Never mind. For others asking the same question, read this article on virtual destructors. I fixed it by adding a "virtual ~MetaTest(void)" declaration and defining it as "MetaTest::~MetaTest(void) { }"

Thanks again,