A simple example of pointer-to-const

What is "const correctness"?

"const correctness" means using the keyword const to prevent const objects from getting mutated.

In this example we show passing by pointer-to-const. In case of pass by pointer-to-const, any attempts to change to the caller's variable within the function would be flagged by the compiler as an error at compile-time. This check is done entirely at compile-time: there is no run-time space or speed cost for the const. Also passing by pointer-to-const is more efficient as it avoids temporary objects.

So here is the example:

//Program tested on Microsoft Visual Studio 2008 - Zahid Ghadialy
//Simple program to show const pointers
#include<iostream>

using namespace std;

class A
{
public:
  int *var;
};

void someFunc(const A* a)
{
  //int b=200;
  //a = &b; - Not Possible to convert int* to const int*
  //          We can use a const_cast but thats not the purpose here
  *(a->var) = 200; //Possible to change value of const pointer
}

void anotherFunc(const A* a)
{
  if(a)
  {
    if (a->var)
      delete a->var;
    delete a;
  }
}

int main()
{
  A *a = new A;
  a->var = new int(1024);
  cout<<"1: a->var = "<<*(a->var)<<endl;
  someFunc(a);
  cout<<"2: a->var = "<<*(a->var)<<endl;
  anotherFunc(a);
  //cout<<"3: a->var = "<<*(a->var)<<endl; - Not Valid
  return 0;
}

The output is as follows: