Pointers, arrays, and datatypes in C++

To run a C++ program:

• Edit a file with any plain-text editor (gedit, emacs, pico/nano, or other), save it with the extension .cpp (for this example assume myprogram.cpp)
• Open a terminal window, use cd command to get to the folder where your C++ files are
• Type g++ myprogram.cpp to compile. If there are error messages, correct the mistake and compile again (note: an old executable a.out will still be there until it is overwritten by a new one. If your program failed to compile and you still try to run it, you would be running the old version).
• After a successful compilation you can run your program by typing a.out on the command line. If you get file not found error, use ./a.out instead.

The example illustrates C++ type system (or a lack of one?)

#include <iostream>  // need this to use the I/O system 
using namespace std; // telling the compiler to use namespace "std",
// where the C++ library is declared.

int main()
{
  int n = 5, m = 3;
  int *p = &n; // p is a pointer to int, points to n
  int *q = &m; // q is a pointer to int, points to m

  *p = 6; // changing n via p
  cout << "n = " << n << endl; // printing n
  q = p; // copied the address from p to q
  cout << "*q = " << *q << endl; // printing what q points to

  m = 6;
  q = &m;

  cout << "Below 1 represents true, 0 represents false " << endl;
  cout << "m == n: " << (m == n) << endl;
  cout << "*p == *q: " << (*p == *q) << endl;
  cout << "p == q: " << (p == q) << endl;

  // Booleans are not a separate type in C++,
  // they are integers: 0 = false, any other value means true
  // Boolean operations are applicable to integers (and chars)
  cout << "true = " << true << endl;
  cout << "false = " << false << endl;
  if (2) cout << "2 means true" << endl;
  if (!0) cout << "0 means false" << endl;
  if ('A') cout << "characters with non-zero ASCII code are true, too" << endl;
  if (! '\0') cout << "end of string character '\\0' means false" << endl;
  cout << (('?' - 6) || !89) << " = ('?' - 6) || !89 - Huh? That's an integer, too?" << endl;

  const int size = 3;
  int A[size];
  // array elements are not initialized
  cout << "Printing uninitialized array..." << endl;
  for (int i = 0; i < size; ++i) {
    cout << A[i] << endl;
  }

  // going through the array using pointers
  for (p = A; p < A + size; p++) {
    *p = 0;
  }

  cout << "Printing the array initialized to 0s..." << endl;
  for (int i = 0; i < size; i++) {
    cout << A[i] << endl;
  }

  // All of these are the same:
  cout << "A[2] = " << A[2] << endl;
  cout << "*(A + 2) = " << *(A + 2) << endl;
  cout << "2[A] = " << 2[A] << endl;

  return 0;

}