Constructor and deconstructor

**ncode** · 06-11-2012

Code:

#include <iostream>

using namespace std;

class Walnut
{
public:
    int Size;
};

class Squirrel
{
private:
    Walnut *MyDinner;

public:
    Squirrel();
    ~Squirrel();
};

Squirrel::Squirrel()
{
    cout << "Starting!" << endl;
    MyDinner = new Walnut;
    MyDinner->Size = 30;
}

Squirrel::~Squirrel()
{
    cout << "Cleaning up my mess!" << endl;
    delete MyDinner;
}

int main()
{
    Squirrel *Sam = new Squirrel;
    Squirrel *Sally = new Squirrel;

    delete Sam;
    delete Sally;

    return 0;
}

I dont understand much about this code. Why the output is Starting! Starting Cleaning up my mess! Cleaning up my mess! ?????
Why it does not (Starting! Cleaning up my mess! Starting! Cleaning up my mess!) Because

Squirrel();
~Squirrel();

It's executed one by one not together?

**laserlight** · 06-11-2012

The default constructor for Squirrel is invoked on this line:

Code:

Squirrel *Sam = new Squirrel;

Then again on this line:

Code:

Squirrel *Sally = new Squirrel;

The destructor for Squirrel is invoked on this line:

Code:

delete Sam;

Then on this line:

Code:

delete Sally;

Hence the output since it is as if you are calling two functions named C and D:

Code:

C();
C();
D();
D();

**Elkvis** · 06-11-2012

it's generating the output you are seeing because you are creating both objects first, then deleting both objects. the constructor gets called when you say new squirrel, and the destructor gets called when you say delete. since you are doing two news, followed by two deletes, you're getting exactly the output that you should.

**ncode** · 06-11-2012

ya i know. But i just cant get it into my head.

I create sam object with squirrel class
inside there are Squirrel and ~Squirrel <-- execute follow sequence
Create sally object with squirrel class
same as 2nd

U know what I mean. I mean like if it's executed according to sequence, it's quite impossible. srry, i try to turn around the theory.

**laserlight** · 06-11-2012

Okay, here's a test. Consider this program:

Code:

#include <iostream>

void bar()
{
    std::cout << "World!" << std::endl;
}

void foo()
{
    std::cout << "Hello!" << std::endl;
}

int main()
{
    foo();
    bar();
}

Now, I tell you that the output of this program will be:

Code:

World!
Hello!

because bar is defined before foo. Do you agree? Why or why not?

**ncode** · 06-11-2012

Isn't the output is
Hello!
World!

Because in main() we execute foo() first then later bar(). Am I right?.......

**laserlight** · 06-11-2012

Originally Posted by ncode

Because in main() we execute foo() first then later bar(). Am I right?

Exactly!

So, why do you say that:

Originally Posted by ncode

inside there are Squirrel and ~Squirrel <-- execute follow sequence

when obviously in main() we execute the constructor first, twice, then later execute the destructor, twice?

**ncode** · 06-11-2012

oh wait. How do the Squirrel() been executed when I didnt perform sam->Squirrel() or sam->~Squirrel(). i will continue 2moro as it get night now.thanks.

**laserlight** · 06-11-2012

Originally Posted by ncode

How do the Squirrel() been executed when I didnt perform sam->Squirrel() or sam->~Squirrel().

Refer to my post #2. I explain just what causes the default constructor and destructor to be executed.

**student111** · 06-11-2012

Can you pls explain
Why has he should define the variable "int size" in private: of the class instead of public:
Have you defined that variable in public: intentionally so that it could be used by the whole program?
Do you want to use that variable as "global variable"?

Thanks

**antred** · 06-11-2012

Originally Posted by ncode

Code:

    Squirrel *Sam = new Squirrel;
    Squirrel *Sally = new Squirrel;

    delete Sam;
    delete Sally;

I'm going to send PETA on your ass!

Really just letting you know that, technically speaking, it's called destructor, not deconstructor.

**R41D3N** · 06-11-2012

To make it simple, A Constructor [Squirrel()] is called when an object to the class is created.

For example,

Code:

 Squirrel *Sam = newSquirrel;
 Squirrel *Sally = newSquirrel;

The destructor [~Squirrel()] is only called when that object is destroyed, which you did in your code by using delete. Till then destructor is just sitting there waiting for your Squirrel to die/delete :P

**Elkvis** · 06-11-2012

Originally Posted by antred

it's called destructor, not deconstructor.

perhaps he's a devin townsend fan

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