Question about containers use

**Mortissus** · 02-15-2005

Hi,

is it correct to use vector, for example, with no dynamically allocated objects?

Code:

#include <iostream>
#include <vector>

using namespace std;

class Mean{
public:
	Mean(int a, int b){ mean = (a+b)/2.0; }
private:
	float mean;
};

int main()
{
	vector<Mean> means;
	for(int i=0 ; i<5 ; i++){
		means.push_back(Mean(i, i+1));
	}
	return 0;
}

I see a problem, correct me if I am wrong, you create two objects in the statment

Code:

means.push_back(Mean(i, i+1));

. The first is created to be passed to the function, and the second, the copy constructor is called.

Must I use pointers instead? Or this is good programming?
Thanks any insight.

**Thantos** · 02-15-2005

The push_back() is prefectly ok. An un-named object of type Mean is created an it's added to the vector. You can use pointers in a vector but from my experience its a lot more trouble then it's worth.

I see nothing wrong with the way you are putting values into the vector.

**Mortissus** · 02-15-2005

Thanks!

**Magos** · 02-15-2005

Just make sure your class has a decent copy constructor (if neccessary) since std::vector may need to copy the data if it grows larger.

**Hunter2** · 02-15-2005

>>since std::vector may need to copy the data if it grows larger.
It would seem to me that you need a copy constructor for adding elements to the vector, and an assignment operator for subsequent resizes. The copy constructor is generated automatically unless you explicitly define one, and so is the assignment operator. In this case you have no problem, since the only data contained in the class is a POD (plain 'ol data) type, float. However, if you have dynamically allocated memory etc. and a destructor that frees it, then you'll have problems unless you define your own copy constructor/assignment operator - or unless you decide to go the route of storing pointers in the container.

**Mortissus** · 02-15-2005

Ok! Thanks!

P.S.: Magos, your link in your signature seems to be broken.

**Magos** · 02-15-2005

hunter2: Yes, a default plain-copy copy constructor is automatically created. What I meant by "decent" were a hand-made one if some dynamic allocation took place.

mortissus: Has been for half a year. Been too lazy to update sigs on all the forums I visit :P

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