Question

Below is a Win32 Console App procedure that demonstrates the dependence of various pointers on an array. A change to the values in the original array (model) by for example uncommenting the lines marked ‘// uncomment …’ results in a change to the output. My question is how do I get or mimic this behaviour in a C# managed code environment (i.e. without using unsafe and pointers)?

#include "stdafx.h"
#include <iostream>
using namespace std;

int _tmain(int argc, _TCHAR* argv[])
{
    float model[100];
    for(int i = 0; i < 100; i++) { model[i] = i; }

    // uncomment these to alter the results
    //model[5] = 5000;
    //model[20] = 20000;
    //model[38] = 38000;

    static const int componentCount = 5;

    float* coefs = model;                   // coefs points to model[0]
    float* mean = coefs + componentCount;   // mean points to model[0 + componentCount] == model[5]
    float* cov = mean + 3*componentCount;   // cov points to model[0 + componentCount + 3*componentCount] == model[20] 

    int ci = 2;
    float* c = cov + 9*ci;  // c points to model[0 + componentCount + 3*componentCount + 9*ci] == model[38] 

    int i = 0;
    cout <<"model : "<< model[i] << endl;   // 0
    cout <<"coefs : "<< coefs[i] << endl;   // 0
    cout <<"mean  : "<< mean[i] << endl;    // 5 (or 5000)
    cout <<"cov   : "<< cov[i] << endl;     // 20 (or 20000)
    cout <<"ci    : "<< ci << endl;         // 2
    cout <<"c     : "<< c[i] << endl;       // 38 (or 38000)

cin.get(); }

Answer 1

You can do the same thing in C# without unsafe code:

struct ArrayPointer<T>
{
    private T[] array;
    private int offset;
    public ArrayPointer(T[] array) : this(array, 0)
    {
    }
    private ArrayPointer(T[] array, int offset)
    {
        Debug.Assert(array != null);
        Debug.Assert(offset >= 0);
        Debug.Assert(offset < array.Length);
        this.array = array;
        this.offset = offset;
    }
    public static ArrayPointer<T> operator+(ArrayPointer<T> p1, int p2)
    {
        return new ArrayPointer<T>(p1.array, p1.offset + p2);
    }

And so on. Define operators for addition, subtraction, increment, decrement, comparison, indexing, conversion from arrays, and so on. Then you can say:

int[] arr = whatever;
ArrayPointer<int> pointer = arr;
pointer+=2;
pointer--;
int x = pointer[3];

and so on.

This approach has a lot of nice properties. For example, you can do a debug assert if you ever compare p1 > p2 when p1 and p2 are pointers to the interiors of different arrays. That is almost always a bug in C, but a hard one to catch.