Initialising arrays with default values in derived class

bobsta · #1 11-Feb-2009, 01:01

Hi,

I am new to C++ having recently migrated from FORTRAN. I am trying to create multiple derived classes each containing various integer or float parameters and 2 arrays, let's call them X and Y, of different sizes (hence different derived classes). X and Y are private data members of the derived class. My problem is that I want to initialise each instance of these derived classes to have default values for XArray and Y, of which X will be overwritten later (using a member function) on the basis of values returned by an external function.
I hope this question is not too banal.

Thanks in anticipation

CPP / C++ / C Code:

//Class defintion for simple derived class FieldA
//Derived from Field
//FieldA has additional private member arrays
//to be initialised
#include <iostream>
using namespace std;

class Field
{
protected:
	int A;
	int B;
	int C;
	int D;
	double E;

public:
	// constructor
	Field();
	~Field();

	//accessors
	void SetC (int c) { C=c;}
	void SetD (int d) { D=d;}
	void SetE (double e) { E=e;}
	int GetA () const {return A;}
	int GetB () const {return B;}
	int GetC () const {return C;}
	int GetD () const {return D;}
	double GetE () const {return E;}
};

class FieldA: public Field
{
private:
	int Y[41]; //arrays
	int X[80]; 

public:
	//constructor and destructor
	// constructor initialises values and arrays
    FieldA():A(40),B(4),
		Y(-80,-76,-72,-68,-64,-60,-56,-52,-48,
		  -44,-40,-36,-32,-28,-24,-20,-16,-12,
		  -8,-4,0,4,8,12,16,20,24,28,32,36,34,
		   40,44,48,52,56,60,64,68,72,76,80),
		X(110,110,110,110,110,110,110,110,
		  110,110,110,110,110,110,110,110,
		  110,110,110,110,110,110,110,110,
		  50,50,50,50,50,50,50,50,
		  80,80,80,80,80,80,80,80,
		  90,90,90,90,90,90,90,90,
		  30,30,30,30,30,30,30,30,
		  110,110,110,110,110,110,110,110,
		  110,110,110,110,110,110,110,110,
		  110,110,110,110,110,110,110,110){};
	//destructor
	~FieldA(){};


};



int main()
{
	FieldA Test;
	cout << "A : " << Test.GetA() << endl;
    cout << "B : " << Test.GetB() << endl;
	
	Test.SetC(0);
	cout << "C : " << Test.GetC() << endl;

	Test.SetD(57);
    cout << "D : " << Test.GetD() << endl;

	Test.SetE(17.32);
	cout << "E : " << Test.GetE() << endl;

	return 0;
}

Mexican Bob · #2 11-Feb-2009, 05:59

Quote:

Originally Posted by bobsta

My problem is that I want to initialise each instance of these derived classes to have default values for XArray and Y, of which X will be overwritten later (using a member function) on the basis of values returned by an external function.

In your Field class, you should utilize its ctor to initialize it to a known good value or state. That is, your members should be initialized to at least zero, if applicable. In your case, they are all POD types.

CPP / C++ / C Code:

//Class defintion for simple derived class FieldA
//Derived from Field
//FieldA has additional private member arrays
//to be initialised

#include <iostream>
#include <cstdlib>

using namespace std;

class Field
{
protected:
    int A;
    int B;
    int C;
    int D;
    double E;
public:
    // constructor
    Field() : A(0), B(0), C(0), D(0), E(0){};
    virtual ~Field(){};
    //accessors
    void SetC (int c) { C=c;}
    void SetD (int d) { D=d;}
    void SetE (double e) { E=e;}
    int GetA () const {return A;}
    int GetB () const {return B;}
    int GetC () const {return C;}
    int GetD () const {return D;}
    double GetE () const {return E;}
};

class FieldA: public Field
{
private:
    int Y[41]; //arrays
    int X[80]; 
public:
    //constructor and destructor
    // constructor initialises values and arrays
    FieldA(){
	int aY[]={-80,-76,-72,-68,-64,-60,-56,-52,-48,
	  -44,-40,-36,-32,-28,-24,-20,-16,-12,
	  -8,-4,0,4,8,12,16,20,24,28,32,36,34,
	   40,44,48,52,56,60,64,68,72,76,80};
	memcpy(Y, aY, sizeof(Y)/sizeof(int));

	int aX[]={110,110,110,110,110,110,110,110,
	  110,110,110,110,110,110,110,110,
	  110,110,110,110,110,110,110,110,
	  50,50,50,50,50,50,50,50,
	  80,80,80,80,80,80,80,80,
	  90,90,90,90,90,90,90,90,
	  30,30,30,30,30,30,30,30,
	  110,110,110,110,110,110,110,110,
	  110,110,110,110,110,110,110,110,
	  110,110,110,110,110,110,110,110};
	memcpy(X, aX, sizeof(X)/sizeof(int));
	A = 40;
	B = 4;
    }
    //destructor
    virtual ~FieldA(){};
};

int main()
{
    FieldA Test;
    cout << "A : " << Test.GetA() << endl;
    cout << "B : " << Test.GetB() << endl;

    Test.SetC(0);
    cout << "C : " << Test.GetC() << endl;

    Test.SetD(57);
    cout << "D : " << Test.GetD() << endl;

    Test.SetE(17.32);
    cout << "E : " << Test.GetE() << endl;

    return 0;

}

...not exactly what you were attempting to do, but at least a working example of what I think it is that you are wanting to accomplish.

If this is some form of classroom assignment, I think that the goal here may be to demonstrate that you can't initialize inherited members or arrays in an "initialization list," though I may be wrong. A review of the standard may be a quick way to find out.

MxB

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Initialising arrays with default values in derived class

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