Making an AbstractOdeSolver class library to solve differential equations numerically (c++) - c++

Question

I am trying to make and ode solver using c++ and numerical methods (euler, heun and runge kutta). first i made and abstract class for ode solving requirements then i made a separate class for each solver inheriting from the ABClass. there is no problem with the code excpet it works only with first order differential equations as dydt = y
however i need to expand it to be able to solve a system of first order differential equations as dydt = x-y & dxdt = y together
here is the header file
#ifndef ABSTRACTODESOLVER_H_INCLUDED
#define ABSTRACTODESOLVER_H_INCLUDED
#include <iostream>
using namespace std;
class AbstractOdeSolver
{
private:
double stepsize;
double initialTime;
double finalTime;
double initialValue;
public:
double (*RHS)(double, double); //pointer to function
void SetStepSize(double h);
void SetIntervalTime(double t0, double t1);
void SetInitialValue(double y0);
void setRHS(double (*pRHS)(double, double)); //set pointer function
double getStepSize(){return stepsize;}
double getInitialTime(){return initialTime;}
double getFinalTime(){return finalTime;}
double getInitialValue(){return initialValue;}
virtual void SolveEquation() = 0;
};
class EulerSolver : public AbstractOdeSolver
{
public:
virtual void SolveEquation();
};
class HeunSolver : public AbstractOdeSolver
{
public:
virtual void SolveEquation();
};
class RungeKuttaSolver : public AbstractOdeSolver
{
public:
virtual void SolveEquation();
};
#endif // ABSTRACTODESOLVER_H_INCLUDED
and this is the source code for one solver:
void EulerSolver::SolveEquation(){
double yNew = 0.0;
double yOld = getInitialValue(); // y0 initial value of Y
double tInit = getInitialTime(); // t0 initial value of T
double tFinal = getFinalTime();
double h = getStepSize();
for (double i = tInit; i <= tFinal; i += h){
yNew = yOld + (h * RHS(tInit,yOld));
yOld = yNew;
tInit += h;
cout << left << setw(5) << tInit << " " << setw(5) << yNew << endl;
}
}
and this is a program:
double Func(double t, double y){
return y;
}
EulerSolver euler1;
euler1.SetIntervalTime(0,0.6);
euler1.SetInitialValue(1);
euler1.SetStepSize(0.1);
euler1.setRHS(Func);
euler1.SolveEquation();
here i am using a function pointer RHS to allow the solver to use the function every time in the iteration, however i need to have more than one function in order to solve higher degree equations and the problem i dont know a way of making two separate functions with related variables (dydt = x-y & dxdt = y) and then having a pointer on each and returning the answer in an array!.
Any ideas will be great.