Vectors (STL Container) - GameCreators Forum

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Michael P

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Location: London (UK)

Posted: 28th Jul 2008 21:20

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I'm learning to use vectors and I'm having trouble with vectors and classes.

In the below code, after the resizing the vector to 50 I would have expected 50 constructions to occur and 0 destructions. However, 1 destructor and 1 constructor occurs. Why is this?

+ Code Snippet

#include <iostream>
#include <vector>

using namespace std;

struct test
{
	string String;

	// Constructor
	test::test()
	{
		cout << "Construction!\n";
	}

	// Destructor
	test::~test()
	{
		cout << "Destruction!\n";
	}
};

void main()
{
	vector<test> Vector;
	Vector.resize(50);

	int Blank;
	cin >> Blank;
}

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Mahoney

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Posted: 28th Jul 2008 21:22

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I imagine it's for efficiency. It should runs the constructor for each as their used.

Windows Vista Home Premium Intel Pentium Dual-Core 1.6 Ghz 1GB DDR2 RAM GeForce 8600GT Twin Turbo

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Michael P

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Posted: 28th Jul 2008 21:29

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I thought that too, but no it doesn't

:

+ Code Snippet

#include <iostream>
#include <vector>

using namespace std;

struct test
{
	string String;

	// Constructor
	test::test()
	{
		cout << "Construction!\n";
	}

	// Destructor
	test::~test()
	{
		cout << "Destruction!\n";
	}
};

void main()
{
	vector<test> Vector;
	Vector.resize(50);
	
	for(int n = 0;n<50;n++)
	{
		Vector[n].String = "Some text...";
	}

	int Blank;
	cin >> Blank;
}

Output is the same as the OP but the string is filled properly.

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Lilith

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Location: Dallas, TX

Posted: 28th Jul 2008 21:39 Edited at: 28th Jul 2008 21:40

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Most like case is that resizing only reserves the space but doesn't fill the slots. If you do a push_back in order to append an item and it needs to re-size the vector for the one item then there's some CPU time taken to do the resize each time. If the size has been increased beyond enough to hold, say, an extra 10 items then it only has to do the resize one time instead of 10 times.

Lilith, Night Butterfly
I'm not a programmer but I play one in the office

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Michael P

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Posted: 28th Jul 2008 22:09 Edited at: 28th Jul 2008 22:09

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I tried using push_back and now I get 1 construction and 50 destructions

. The code is below.

+ Code Snippet

#include <iostream>
#include <vector>
#include <string> 
using namespace std;

struct test
{
	string String;
	int * Integer;

	// Constructor
	test::test()
	{
		cout << "Construction!n";
	}

	// Destructor
	test::~test()
	{
		cout << "Destruction!n";
	}
}Stuff;

void main()
{
	vector<test> Vector;
	
	for(int n = 0;n<50;n++)
	{
		Stuff.String = "Some text...";
		Vector.push_back(Stuff);
	}

	int Blank;
	cin >> Blank;
}

The reason why I care about the constructions and destructions is that I want to be able to allocate memory dynamically during the construction and deallocate memory during the destruction. Here is some code which is an example of what I want to do (unsurprisingly it does not work):

+ Code Snippet

#include <iostream>
#include <vector>
#include <string> 
using namespace std;

struct test
{
	int * Integer;

	// Constructor
	test::test()
	{
		cout << "Construction!n";
		Integer = new int;
	}

	// Destructor
	test::~test()
	{
		cout << "Destruction!n";
		delete Integer;
	}
};

void main()
{
	vector<test> Vector;
	Vector.resize(50);
	
	for(int n = 0;n<50;n++)
	{
		*Vector[n].Integer = n;
	}

	for(int n = 0;n<50;n++)
	{
		cout << *Vector[n].Integer << ".n";
	}
	int Blank;
	cin >> Blank;
}

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IanM

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Location: In my moon base

Posted: 28th Jul 2008 23:45 Edited at: 28th Jul 2008 23:46

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What is happening is that a single instance of the class is created (as you've seen), then the rest are created using the copy constructor (the default one as you haven't provided one).

+ Code Snippet

#include <iostream>
#include <vector>

using namespace std;

struct test
{
        string String;

        // Constructor
        test::test()
        {
                cout << "Construction!n";
        }

        // Copy Constructor
        test::test(const test&)
        {
                cout << "Copied!n";
        }

        // Destructor
        test::~test()
        {
                cout << "Destruction!n";
        }
};

int main()
{
        vector<test> Vector;
        Vector.resize(50);

        int Blank;
        cin >> Blank;
}

[EDIT]It's 'reserve' that allocates memory but doesn't construct the objects - the space should be treated as inaccessible.

[b]Utility plug-ins collection (updated 01/07/08) and
http://www.matrix1.demon.co.uk

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Michael P

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Location: London (UK)

Posted: 30th Jul 2008 12:08

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Thanks, but I'm still a bit confused. In the below code I'm trying to increase the size of the vector by one without losing all the previously added data. How can this be done?

+ Code Snippet

#include <iostream>
#include <vector>

using namespace std;

struct test
{
		int Integer;

        // Constructor
        test::test()
        {
                //cout << "Construction!n";
        }

        // Copy Constructor
        test::test(const test&)
        {
                //cout << "Copied!n";
        }

        // Destructor
        test::~test()
        {
               //cout << "Destruction!n";
        }
};

int main()
{
        vector<test> Vector;
        Vector.resize(50);

		for(int n = 0;n<50;n++)
		{
			Vector[n].Integer = n;
		}
		
		cout << "Before increasing size by 1:\n";
		for(int n = 0;n<50;n++)
		{
			cout << Vector[n].Integer << ",";
		}

		Vector.resize(51);

		cout << "\n\nAfter increasing size by 1:\n";
		for(int n = 0;n<50;n++)
		{
			cout << Vector[n].Integer << ",";
		}

        int Blank;
        cin >> Blank;
}

Output:

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Mahoney

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Posted: 30th Jul 2008 18:40

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That number is the value of an int that isn't set to any value.

Windows Vista Home Premium Intel Pentium Dual-Core 1.6 Ghz 1GB DDR2 RAM GeForce 8600GT Twin Turbo

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Charles Thompson

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Posted: 30th Jul 2008 19:07

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Just out of curiosity sakes set the size of the vector to 50 originally and perform that same code. If it works then I have a theory on why your current code doesn't. Let me know how it goes.

Charles Thompson

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Michael P

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Location: London (UK)

Posted: 30th Jul 2008 20:08

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I realized what the problem was; the copy constructor wasn't doing anything.

In the below code, the resize operation will properly allocate new memory to new elements. Let me know if you see anything wrong with this:

#include <iostream>
#include <vector>

using namespace std;

struct test
{
		int * Integer;

// Constructor
        test::test()
        {
			Integer = new int;
            //cout << "Construction!n";
        }

// Copy data into the new memory
			*Integer = *copyme.Integer;

//cout << "Copied!\n";
        }

// Destructor
        test::~test()
        {
			delete Integer;
            //cout << "Destruction!n";
        }
};

int main()
{
		// Create vector
        vector<test> Vector;

// Resize it to 50 (first resize)
        Vector.resize(50);

// Assign all elements with a value
		for(int n = 0;n<50;n++)
		{
			*Vector[n].Integer = n;
		}
		
		// Output data
		cout << "Before increasing size by 1:\n";
		for(int n = 0;n<50;n++)
		{
			cout << *Vector[n].Integer << ",";
		}
		
		// Increase size by 1 and assign a value to the new element
		Vector.resize(51);
		*Vector[50].Integer = 1337;

// Output data
		cout << "\n\nAfter increasing size by 1:\n";
		for(int n = 0;n<51;n++)
		{
			cout << *Vector[n].Integer << ",";
		}
		
		// Wait for input before exit
        int Blank;
        cin >> Blank;
		return(0);
}

+ Code Snippet

#include <iostream>
#include <vector>

using namespace std;

struct test
{
		int * Integer;

        // Constructor
        test::test()
        {
			Integer = new int;
            //cout << "Construction!n";
        }

        // Copy Constructor
        test::test(const test ©me)
        {
			// Assign memory to pointer
			Integer = new int;

			// Copy data into the new memory
			*Integer = *copyme.Integer;

            //cout << "Copied!\n";
        }

        // Destructor
        test::~test()
        {
			delete Integer;
            //cout << "Destruction!n";
        }
};

int main()
{
		// Create vector
        vector<test> Vector;

		// Resize it to 50 (first resize)
        Vector.resize(50);

		// Assign all elements with a value
		for(int n = 0;n<50;n++)
		{
			*Vector[n].Integer = n;
		}
		
		// Output data
		cout << "Before increasing size by 1:\n";
		for(int n = 0;n<50;n++)
		{
			cout << *Vector[n].Integer << ",";
		}
		
		// Increase size by 1 and assign a value to the new element
		Vector.resize(51);
		*Vector[50].Integer = 1337;

		// Output data
		cout << "\n\nAfter increasing size by 1:\n";
		for(int n = 0;n<51;n++)
		{
			cout << *Vector[n].Integer << ",";
		}
		
		// Wait for input before exit
        int Blank;
        cin >> Blank;
		return(0);
}

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haYnguy

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Location: on my island

Posted: 1st Aug 2008 07:35

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i was gonna say. The copy constructor hasn't been defined.

-- from my island, aloha wau 'ia 'oe --

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