destructors

[geeksquad007]

Just wondering about the use of destructors.

Eg:

file: mainclass.h
class name
{
    public:
        name();
        ~name();
        int removeall();
...

};

file: classimplementation.cpp

#include "mainclass.h"

name::name()
{
...
}

name::~name()
{....}

int name::removeall()
{
    ~name();  just wondering why it doesn't work. is there anything i am missing?
    return 1;
}
 

[tath]

you might want to change ~name(); to name::~name(); or this->~name(); at line 27

[geeksquad007]

Thank you for the reply. It still doesn't work. any other ideas?

only works when i am outside the class, create class variable and do:

name data;

data.~name();

Thanks for editing . I think "this" worked.

[Peter87]

The destructor will be called automatically when the object is destroyed. No need to call it explicitly.

[geeksquad007]

it will be called automatically but it would be kind of a shortcut if one needs to remove all datas and keep the program running. I actually used it in some programs before but just couldn't remember how i used it.

[MiiNiPaa]

If you created object using new, you can use delete to destroy it.
In other cases it will be called automaticlly, as Peter87 said.

[MikeyBoy]

The sensible thing to do is to have a method that performs whatever cleanup tasks you want to be able to do. Then the destructor can call that mthod, and your removeall method can also call it.

[ne555]

Invoking the destructor twice (one in the `removeall()' and later implicit when it goes out of scope)
results in undefined behaviour

[MiiNiPaa]

I suppose he meant to have clear_all() method which both destructor and user function will call.

[xkcd83]

Yes. So not call destructor explicitly.
This whole set of FAQ might be more enlightening:
http://www.parashift.com/c++-faq/dont-call-dtor-on-local-really.html

[geeksquad007]

Thank you for all the information. And Miniipaa, yes thats true but can i not make that function and just have all the things in destructor and just call destructor.

Actually i am not calling the destructor twice. But only trying it explicitly i guess. It does the garbage dump only when there is something to be destroyed. Simple eg:

Name::~name()
{
    If(head)
         .......
}

So since there is constructor, the destructor will be called once automatically but if i call it explicitly, then it wont be called again as there is nothing to be destroyed.

[MiiNiPaa]

Destructor does more than you writing, and you can't change what it will do. Non-pointer data types, service information... So some memory areas will be freed twice, and imagine what happens if some other data was allocated here?

[Stewbond]

If your method: removeall() is supposed to have similar functionality to a destructor (as in it will free allocated memory), then in your case, I think it would be better to call removeall() from the destructor instead of vice versa.

name::~name()
{
    removeall();
}

int name::removeall()
{
    delete someMember;
    return 1;
}

This is similar to what happens in std::ifstream's constructor:

ifstream::ifstream(string filename)
{
    open(filename);
}
void ifstream::open(string filename) { ... }

The constructor and open have similar functionality. Instead of calling the constructor from open(), they call open() from the constructor. That will let us make use of the code reuse.

I actually commonly use this sort of thing when I making a controller that needs to be totally reset:

class PID
{
protected:
    init(); // Allocates memory
    exit(); // De-allocates memory
public:
    PID() { init(); }
    ~PID() { exit(); }
    Reset() { exit(); init(); }
};

The other advantage of putting your code inside of methods instead of constructors or destructors is that ctor/dtors are not inherited. If you want someone to inherit this functionality, then keep that code out of a constructor/destructor.

The only disadvantage that I see is that you can't make use of the initialization lists this way and you have to rely on assignments.

Edit: Thanks ne555

[ne555]

Well, they are initializated already.

By the way, your `Reset()' is backwards.

[geeksquad007]

Thank You everyone. That really helped!