Functions

As an example, we’ll write a function to determine the factorial of a given number. 5 Factorial is: 1 * 2 * 3 * 4 * 5 = 120

#include <iostream>
using namespace std;

int factorial(unsigned int value){
  unsigned int result = 1;
  while(value > 1){
    result *= value--;
  }
  return result;
}

int main(){
  cout << "Factorial 5 is: " << factorial(5) << endl;
  return 0;
}

In C++, variables have scope and objects have lifetimes. It is important to understand both of there concepts.

• The scope of a name is the part of the program’s text in which that name is visible.
• The lifetime of an object is the time during the program’s execution that the object exists.

Another important aspect about functions are arguments, when a parameter is a reference, we say that its corresponding argument is passed by reference a reference parameter is an alias for the object to which it is bound; that is, the parameter is an alias for its corresponding argument.

When the argument value is copied, the parameter and argument are independent objects. We say such argument are passed by value.

Pointer Parameter

#include <iostream>
using namespace std;

void reset(unsigned int *p){
  cout << "reseting value" << endl;
  *p = 0;
}

int main(){
  unsigned int i = 40;
  cout << "address i: " << &i << endl;
  cout << "value i: " << i << endl;
  reset(&i);
  cout << "value i: " << i << endl;
  cout << "address i: " << &i << endl;
}

output

address i: 0x7ffe760a9944
value i: 40
reseting value
value i: 0
address i: 0x7ffe760a9944

By reference

#include <iostream>
using namespace std;

void reset(unsigned int &reference){
  cout << "reseting value" << endl;
  reference = 0;
}

int main(){
  unsigned int i = 40;
  cout << "address i: " << &i << endl;
  cout << "value i: " << i << endl;
  reset(i);
  cout << "value i: " << i << endl;
  cout << "address i: " << &i << endl;
}

output

address i: 0x7ffcf264e074
value i: 40
reseting value
value i: 0
address i: 0x7ffcf264e074

By Value

#include <iostream>
using namespace std;

void reset(unsigned int parameter){
  cout << "changing value" << endl;
  parameter = 0;
}

int main(){
  unsigned int i = 40;
  cout << "address i: " << &i << endl;
  cout << "value i: " << i << endl;
  reset(i);
  cout << "value i: " << i << endl;
  cout << "address i: " << &i << endl;
}

output

address i: 0x7ffd56db2444
value i: 40
reseting value
value i: 40
address i: 0x7ffd56db2444

Reference return

Whether a function call is an lvalue depends on the return type of the function. Calls to functions that return references are lvalues.

#include <iostream>
using namespace std;

char &get_val(string &s, unsigned int index)
{
  return s[index];
}

int main()
{
  string s = "this is a value";
  cout << s << endl;
  get_val(s,0) = 'T';
  cout << s << endl;
  return 0;
}

The return value is a reference.

Output

this is a value
This is a value

Default Arguments

Some functions have parameters that are given a particular value in most, but not all calls. For example if we create a window we might want a particular height, width and title, also we might want to allow users to pass values other than defaults.

#include <gtkmm.h>
using namespace std;

void screen(unsigned int width = 640, unsigned int height = 480, string title = "GTK Window"){
  Gtk::Window window;
  window.set_default_size(width, height);
  window.set_title(title);

  Gtk::Main::run(window);
}

int main (int argc, char *argv[])
{
  Gtk::Main kit(argc, argv);
  screen();
  return 0;
}

If we want to use the default argument, we omit that argument when we call the function.

screen();     // Equivalent to screen(640, 480, "GTK Window")
screen(100);  // Equivalent to screen(100, 480, "GTK Window")
screen(100,200, "My Window"); // Different values than default

Tu run this example:

g++ window.cpp -o window.o `pkg-config --libs --cflags gtkmm-2.4`

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