Creating Array of Classes

Question

I got a class such as:

class me362
{
public:
    void geometry(long double xLength);
    void mesh(int xNode);
    void properties(long double H, long double D, long double K,long double Q, long double DT,long double PHO,long double CP, long double TINF);
    void drichlet(long double TLeft,long double TRight);
    void neumann(bool Tlinks, bool Trechts);
    void updateDiscretization(long double**** A,long double* b, long double* Tp);
    void printVectorToFile(long double *x);
private:
    int xdim;
    long double xlength;
    long double tleft;
    long double tright;
    long double h;
    long double d;
    long double k;
    long double q;
    long double dt;
    long double cp;
    long double rho;
    long double Tinf;
    bool tlinks;
    bool trechts;
};

And I initialize it using

me362 domain1;
me362 domain2;
me362 domain3;

But I want to determine the number of domains that I want to initialize. So I need a dynamic array of me362 structures. How can I do that? Can it be done?

Thank you all,

Emre.

What nsarray tag has to do with C++ ? It's for Objective-C. — Mahesh
– Mahesh, Commented Apr 30, 2011 at 10:46
@Alexandre C. - yeah but I have to =) It's for implementing coupled partial differential equation discretizations. Need to deal with very large arrays.. — Emre Turkoz
– Emre Turkoz, Commented Apr 30, 2011 at 12:12

Mahesh · Accepted Answer · 2011-04-30 10:40:41Z

3

Yes, it can be done. Use std::vector instead which increases it's size dynamically on every push_back operation.

std::vector<me362> obj ;

for( int i = 0; i < numberOfInstancesRequired; ++i )
{
    obj.push_back( me362() ) ;
}

edited Apr 30, 2011 at 10:40

answered Apr 30, 2011 at 10:35

Mahesh

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4 Comments

adl Over a year ago

I known the above code is just an example, but in this particular example where the number of instances is known, it will be much more efficient to call obj.reserve(numberOfInstancesRequired); right before the loop.

Emre Turkoz Over a year ago

@adl I actually want the user to enter the number of instances. So it's unknown. But thank you anyway

KillianDS Over a year ago

Just use std::vector<me362> myArray(numberOfInstancesRequired, me362()); . This will both do the size reservation and creating this many objects without the need for a push_back loop.

adl Over a year ago

@Emre: this value does not need to be a constant known at compile-time. If the user gives a value, you now that value at run-time and you can call reserve() or use the syntax of KillianDS.

Lstor · Accepted Answer · 2011-04-30 10:50:44Z

1

Use std::vector, which handles dynamic memory for you:

#include <vector>

// ...

std::vector<me362> domains;

std::vector also has a lot of nice features and guarantees, like being layout-compatible with C, having locality of reference, zero overhead per element, and so on.

Also note that std::vector has a constructor that takes an integral argument, and creates that many elements:

// Will create a vector with 42 default-constructed me362 elements in it
std::vector<me362> domains(42);

See any standard library reference (like cppreference.com or cplusplus.com) for details about using std::vector.)

edited Apr 30, 2011 at 10:50

answered Apr 30, 2011 at 10:34

Lstor

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1 Comment

Emre Turkoz Over a year ago

thank you. I will use the std::vector with a loop to do that.

Eric Z · Accepted Answer · 2011-04-30 10:46:04Z

1

For starters, welcome to the world of STL(standard template library)!

In your case, you can use std::vector, as it can hold variable number of elements for you.

#include<vector>
using namespace std;

//Create a std::vector object with zero size
vector<me362> myVector;

//Insert new items
myVector.push_back(domain1);
myVector.push_back(domain2);
myVector.push_back(domain3);

//Get the size of the vector, i.e., number of elements in vector
myVector.size();

Besides, you can create a vector object like this.

//nMe362: number of elements in vector, me362Obj: init value of each me362 object
vector<me362> myVector(nMe362, me362Obj);

edited Apr 30, 2011 at 10:46

answered Apr 30, 2011 at 10:34

Eric Z

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1 Comment

Emre Turkoz Over a year ago

Thank you I will "push_back" them using a loop as Mahesh described above.

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