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Why is destructor called twice?

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Join Date: Feb 2005

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winbatch

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Why is destructor called twice?

Oct 17th, 2005

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#include <vector>
#include <iostream>
using namespace std;
class Test
{
		public:
		Test()
		{
				cout<<"In TEST constructor"<<endl;
		}
		~Test()
		{
				cout<<"In TEST destructor"<<endl;
		}
 
};
int main()
{
		Test t1;
		vector<Test> vec;
		vec.push_back( t1 );
		cout<<"About to clear vector"<<endl;
		vec.clear();
 
 
}
#include <vector>
#include <iostream>
using namespace std;
class Test
{
		public:
		Test()
		{
				cout<<"In TEST constructor"<<endl;
		}
		~Test()
		{
				cout<<"In TEST destructor"<<endl;
		}
 
};
int main()
{
		Test t1;
		vector<Test> vec;
		vec.push_back( t1 );
		cout<<"About to clear vector"<<endl;
		vec.clear();
 
 
}

I suspect I'm missing something fundamental. When I run this, I get the cout for the destructor twice. (Why would clearing the vector cause the destructor of Test to be called?)

./Temp
In TEST constructor
About to clear vector
In TEST destructor
In TEST destructor

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Join Date: Apr 2004

Posts: 4,461

Reputation: Dave Sinkula has a brilliant future

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Dave Sinkula Dave Sinkula is offline

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long time no c

Re: Why is destructor called twice?

Oct 17th, 2005

A guess:

Help with Code Tags

(Toggle Plain Text)

#include <vector>
#include <iostream>
using namespace std;
class Test
{
		public:
		Test()
		{
				cout<<"In TEST constructor"<<endl;
		}
		~Test()
		{
				cout<<"In TEST destructor"<<endl;
		}

};
int main()
{
		Test t1;
		vector<Test> vec;
		vec.push_back( t1 );
		cout<<"About to clear vector"<<endl;
		vec.clear();
      cout << "done\n";
}

/* my output
In TEST constructor
About to clear vector
In TEST destructor
done
In TEST destructor
*/

The constructor code is the construction of t1. Then a copy constructor is used when it is pushed back on the vector. When the clear() is called, it calls the destructor for the object in the vector. Then t1's destructor is called when it goes out of scope.

"One of the methods used by statists to destroy capitalism consists in establishing controls that tie a given industry hand and foot, making it unable to solve its problems, then declaring that freedom has failed and stronger controls are necessary." --Ayn Rand

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Join Date: Feb 2005

Posts: 466

Reputation: winbatch is on a distinguished road

Solved Threads: 18

winbatch

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Re: Why is destructor called twice?

Oct 17th, 2005

Makes sense. To test, I made it a vector of Test * and only got one destruction call:

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#include <vector>
#include <iostream>
using namespace std;
class Test
{
		public:
		Test()
		{
				cout<<"In TEST constructor"<<endl;
		}
		~Test()
		{
				cout<<"In TEST destructor"<<endl;
		}
};
int main()
{
		Test t1;
		vector<Test *> vec;
		vec.push_back( &t1 );
		cout<<"About to clear vector"<<endl;
		vec.clear();
		cout<<"done"<<endl;
 
}
#include <vector>
#include <iostream>
using namespace std;
class Test
{
		public:
		Test()
		{
				cout<<"In TEST constructor"<<endl;
		}
		~Test()
		{
				cout<<"In TEST destructor"<<endl;
		}
};
int main()
{
		Test t1;
		vector<Test *> vec;
		vec.push_back( &t1 );
		cout<<"About to clear vector"<<endl;
		vec.clear();
		cout<<"done"<<endl;
 
}

./Temp
In TEST constructor
About to clear vector
done
In TEST destructor

----------------
I would imagine then that if my classes are large that I would be much better served (performance wise) to be holding pointers to my objects than copies of my objects....

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Join Date: Feb 2005

Posts: 466

Reputation: winbatch is on a distinguished road

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winbatch

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Re: Why is destructor called twice?

Oct 17th, 2005

[edit](Never mind)

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Join Date: Apr 2004

Posts: 4,461

Reputation: Dave Sinkula has a brilliant future

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Dave Sinkula Dave Sinkula is offline

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long time no c

Re: Why is destructor called twice?

Oct 17th, 2005

I can't help but wonder about your intent with pushing an address. Let's say you were reading records from a file into a temporary line of text. You could avoid the constructor and pass only a pointer to the temporary text, but you wouldn't be keeping unique copies of the data that you'd read. In the end you'd have a pile of pointers all pointing to something that is no longer there.

Be careful with what you are doing, and try to avoid premature optimization. Perhaps you could provide a little more of the 'big picture'; but in the end be careful.