pointer to member?

[ihvc4]

I have the 10 classes car and a member of this class is the integer weight
how can i create an aray of the ineger weight with ponters to that member? car

[kc0arf]

Hello,

If I understand you correctly, you have 10 categories (called cars), and you want to collect data on the car.

You can use a struct to handle this, or make a simple standard that says the 1 storage unit will be the 1 car class, the second storage unit the second class... and go from there.

My guess is that you are going to want to store more than the car's weight.

Make a struct

struct data
{
string carname;
int weight;
}

And then you can make an array:

struct data CarData[10];

And then you can refrence them as you need them:

CarData[3].weight= the value of the weight of the third car.

Give it a shot, and post your code to see if you have it all.

Christian

[glSuccinct]

Off the top of me head:

#include <iostream>
#include <conio.h>

using std::cout;
using std::endl;

struct TData
{
	float fWeight;
	float fLength;
};

typedef float (TData::*TDataFloatPointer); // pointer to member

float CalcAverage(const TData* pArray, int iSize, TDataFloatPointer p)
{
	float fAverage = 0;
	for(int i = 0; i < iSize; ++i)
	{
		float f = pArray->*p; // derefernce through the pointer to member
		fAverage += f;
		++pArray;
	}

	return fAverage/iSize;
}

void PrintArrayData(const TData* pArray, int iSize, TDataFloatPointer p)
{
	float fAverage = CalcAverage(pArray, iSize, p);

	for(int i = 0; i < iSize; ++i)
		cout << pArray++->*p << endl;

	cout << "Average:" << endl << fAverage << endl;
}

int main()
{
	const TData vData[] =
	{
		{ 1.0f, 10.0f },
		{ 2, 9 },
		{ 3, 8 },
		{ 4, 7 },
		{ 5, 6 },
		{ 6, 5 },
		{ 7, 4 },
		{ 8, 3 },
		{ 9, 2 },
		{ 10, 1 }
	};
	const NUM_DATA = sizeof(vData)/sizeof(vData[0]);

	cout << "Weights:" << endl;
	PrintArrayData(vData, NUM_DATA, &TData::fWeight); // get the pointer to member

	cout << "Length:" << endl;
	PrintArrayData(vData, NUM_DATA, &TData::fLength);

	getch();
}

Using a pointer to member construct, this will collect each member's weight or length and return an average.

Pointers to members are a niche type of programing mechanism. There are very few instances where pointers to members are even useful, but the times they are, they're invaluable. They're most useful, in my experince, in certain forms of automation, such as running the same algorithm over different members of the same type. Thus particular example could be boiled down to about 10 lines of code were I to go further w/ the pointers to members and templates, but this is a much more clear example.

Hope it helps!

[glSuccinct]

Ooh, forgot to mention.

In 9 out for 10 times, if you know what types of data you're going to be working on, it's easier just to do like kc0arf said, and just dereference manually.

The only reasons for pointers to members are when you cannot guarantee that you know anything about the source objects (i.e. templates: typedef T1 (TBase::*TMemberPointer);), or you're running an algo on many parts of it.