Question

How can I change the following code such that I can use new and delete instead of malloc and free?

#include <iostream>
#include <cstdlib>
#include <cstdio>
using namespace std;

class myclass{
  private:
    float **m_R;
  public:
    myclass();
    ~myclass();
    float **getR(void);
    void setR(void);
};

myclass::myclass()
{
  // first allocate rows (for pointers)
  m_R = (float**)malloc(3*sizeof(float));
  // next allocate columns for float values
  for(int i=0;i<3;i++)
    *(m_R+i) = (float*)malloc(3*sizeof(float));
}

myclass::~myclass()
{
  // first free memory allocated by columns
  for(int i = 0; i < 3; i++)
  {
    free(m_R[i]);
  }
  // next free memory allocated by rows
  free(m_R);
}

void myclass::setR(void)
{
  for(int i=0;i<3;i++)
  {
    for(int j=0;j<3;j++)
    {
      m_R[i][j] = 10*i+j;
      //cout << m_R[i][j] << ", ";
    }
    //cout << "\n";
  }
}

float **myclass::getR(void)
{
  return m_R;
}

int main () {

  myclass obj;
  obj.setR();

  for(int i=0;i<3;i++)
  {
    for(int j=0;j<3;j++)
    {
      printf("%02d, ",(int)obj.getR()[i][j]);
    }
    cout << "\n";
  }

  return 0;
}

Edit1: Please note that I have to use a function (not written by me) which takes float** as argument and I have no choice (such as vector) than using float**.

Edit2: The function is from Proximity Query Package (PQP) written as following:

int 
PQP_Distance(PQP_DistanceResult *result, 
             PQP_REAL R1[3][3], PQP_REAL T1[3], PQP_Model *o1,
             PQP_REAL R2[3][3], PQP_REAL T2[3], PQP_Model *o2,
             PQP_REAL rel_err, PQP_REAL abs_err,
             int qsize = 2);

Answer 1

• T* a = (T*)malloc(sizeof(T)) becomes new T.
• T* b = (T*)malloc(N * sizeof(T)) becomes new T[N].
• free(a) becomes delete a.
• free(b) becomes delete[] b.

So you get:

myclass::myclass()
{
  // first allocate rows (for pointers)
  m_R = new float*[3];
  // next allocate columns for float values
  for(int i=0;i<3;i++)
    *(m_R+i) = new float[3];
}

myclass::~myclass()
{
  // first free memory allocated by columns
  for(int i = 0; i < 3; i++)
  {
    delete[] m_R[i];
  }
  // next free memory allocated by rows
  delete [] m_R;
}

Note that this is actually not very optimal. If you want 3×3 matrices, you’re better off allocating 9 floats in one block.

Also note that your C is incorrect.

m_R = (float**)malloc(3*sizeof(float));

should be

m_R = (float**)malloc(3*sizeof(float*));

Your code probably “works” because you’re compiling for 32-bit, where float and float* are 4 bytes. On a 64 bit build, float* is 8 bytes.

---

Honestly, though, since your dimensions are fixed and small, you should store everything in the object itself:

class myclass{
  private:
    float m_R[3][3];
  public:
    myclass() {}
    ~myclass() {}
    void setR(void);
    float* operator[](unsigned i) { return m_R[i]; }
    const float* operator[](unsigned i) const { return m_R[i]; }
};

void myclass::setR(void)
{
  for(int i=0;i<3;i++)
  {
    for(int j=0;j<3;j++)
    {
      (*this)[i][j] = 10*i+j;
      //cout << (*this)[i][j] << ", ";
    }
    //cout << "\n";
  }
}

int main () {

  myclass obj;
  obj.setR();

  for(int i=0;i<3;i++)
  {
    for(int j=0;j<3;j++)
    {
      printf("%02d, ",(int)(obj[i][j]));
    }
    cout << "\n";
  }

  return 0;
}

This is how I usually do it, so I get the readability of [][] with the efficiency of in-object storage.