Question

I am having a bit of trouble with C++ classes. I am writing a class (cSpline) to do a cubic spline fit on a set of input points. I want to use a cSpline object within another class (ss304, which provides properties of type 304 stainless steel as a function of temperature). I have four files. Here is the code I have come up with:

cSpline.h:

class cSpline {
private:
  double *setCoeffs(double* x_in, double* f_in, int size);
  double *coeffs;
  double *f;
  double *x;
  int sizex;
public:
  double *interpolate(double* y, int sizey);
  cSpline(double* x_in, double* f_in, int size);
};

cSpline.cpp:

cSpline::cSpline(double* x_in, double* f_in, int size) {
  f = f_in;
  x = x_in;
  sizex = size;
  coeffs = setCoeffs(x, f, size);
}

double* cSpline::setCoeffs (double* x_in, double* f_in, int size) {
  double *ypp = new double[size];
  for (int i = 0; i < size; i++) {
    ypp[i] = 0.0;
  }
  return ypp;
}

double* cSpline::interpolate(double* y, int sizey){
  double *g = new double[sizey];
  for (int i = 0; i < sizey; i++) {
    g[i] = 0.0;
  }
  return g;
}

ss304.h:

#include "cSpline.h"

class SS304 {
private:
    // Material constants
  static const double T0 =     273.0; // standard temp, K
  static const double rho0 =  7924.0; // density at STP, kg/m^3
  static const double Tm =    1700.0; // melting temp, K
  static const double Hm =  265.3+03; // heat of melting, J/kg
  static const double rhom =  7015.0; // density of molten phase at Tm (iron), kg/m^3/K
  static const double drdt =  -0.883; // temperature coefficient of densiy at Tm, kg/m^3/K
  static const double Tv =    3100.0; // vaporization temperature, K
  static const double Hv = 6.258e+06; // heat of vaporization, J/kg
    // Property Array Sizes
  static const int Na1 = 10;
    // Property Arrays
  double alpha1[Na1];   //thermal expansivity, T0 < T < Tm, 1/K
  double Talpha1[Na1];
  cSpline csalpha1(double* x, double* f, int size);
public:
  double* alpha;
  void setProp1D(double* T, int size);
  SS304();
};

ss304.cpp:

#include "ss304.h"

SS304::SS304() {
  double alpha1[Na1]  = {13.6e-6, 16.1e-6, 17.15e-6, 17.8e-6, 18.65e-6, 19.2e-06, 19.85e-06, 20.55e-06, 20.9e-06};
  double Talpha1[Na1] = {   200.,    400.,     500.,    600.,     700.,     800.,     1000.,     1200.,    1400.};

  cSpline csalpha1(Talpha1, alpha1, Na1);
}

void SS304::setProp1D(double* T, int size) {
  double* alpha = new double[size];
  alpha[0] = csalpha1.interpolate(T[0]);
}

What I am trying to accomplish here is this: Upon creation of a ss304 object, I set the properties of 304 stainless, in this case alpha1, at a given set of temperatures, Talpha1. Then I create a cSpline object, csalpha1, for later use. Creation of the cSpline object goes ahead and calculates the spline coefficients. Then, when I call SS304::setProp1D with an array of temperatures, it should set the values of alpha[] based on an interpolation of the cubic spline at each temperature in T[]. Obviously, I left out the full implementation of the spline for the sake of space, but the implementation is irrelevant to the error I get, which is:

ss304.cpp: In member function ‘void SS304::setProp1D(double*, int)’:

ss304.cpp:12: error: ‘((SS304*)this)->SS304::csalpha1’ does not have class type

So, I think I have some basic misunderstanding of how exactly classes work in C++. I think I am trying to use them as I do in Python, where I have this working just fine. But, obviously I am missing something in C++. I have googled around quite a bit, but not found any help that I understood. Any help would be greatly appreciated.

Answer 1

You can’t initialize a data member in the constructor by calling the data member’s constructor. You will need an assignment or initialization method for the object:

SS304::SS304()
{
    static const double alpha1[Na1]  = {13.6e-6, 16.1e-6, 17.15e-6,
                                        17.8e-6, 18.65e-6, 19.2e-06,
                                        19.85e-06, 20.55e-06, 20.9e-06};
    static const double Talpha1[Na1] = {200.,    400.,     500.,
                                        600.,    700.,     800.,
                                       1000.,   1200.,    1400.};
    csalpha1.initialize(Talpha1, alpha1, Na1);
} 

Also, familiarize yourself with initialization lists and the C++ FAQ (which can be found by searching the web).

I converted the arrays into static const to force the compiler to place the data into Read-Only Memory and to access it from the Read-Only Memory (versus copying the data onto the stack).

The compiler will use a default or empty constructor when creating data members of a class.