Parameter passing 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 parameter passing in (old style) C++

#include <iostream>  // need this header file to support the C++ I/O system
using namespace std; // using the standard namespace "std",

// function declarations - needed for the program to compile
void by_value(int n);
void by_reference(int *p);
void swap(int *p, int *q);

int main()
{
  int n = 2, m = 1;
  int *p = &n;
  int *q = &m;

  // passing an integer by value
  by_value(n);
  cout << n << endl;
  by_value(*p); // cannot pass just p
  cout << n << endl;

  by_reference(&n); // cannot pass just n
  cout << n << endl;
  by_reference(p);
  cout << n << endl;

  // swaping n and m
  swap(&n, &m);
  cout << "n = " << n << " m = " << m << endl;

  swap(p, q);
  cout << "n = " << n << " m = " << m << endl;

  return 0;
}

void by_value(int k) {
  k = k + 1;
}

void by_reference(int *r) {
  *r = *r + 1;
}

void swap(int *first, int *second) {
  int temp = *first;
  *first = *second;
  *second = temp;
}