Defining function arguments

I am reading on this site and they have this code

Code:

---

HWND Create(HINSTANCE hinst, LPCTSTR clsname,
                  LPCTSTR wndname, HWND parent = NULL,
                    DWORD dStyle = WS_OVERLAPPEDWINDOW,
                  int x = CW_USEDEFAULT, int y = CW_USEDEFAULT,
                  int width = 450, int height = 380);

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HWND parent = NULL

? you can assign a variable in the functions declaration? How does this work?

It is a default value. If the HWND parameter is not passed, then it defaults to NULL.

Here is another example:

void foo(int x = 10);

You can call the function foo as:
foo(15);
or
foo();

In the second case, x is given the default value of 10.

..

I have been programming for 2-3 years now... and I have wondered about this forever...

I cant believe I didnt know this, thank you.

[edit]
I also noticed this

BOOL CanShow = ::ShowWindow(hwnd, dCmdShow);

What is the :: prefix?

I am reading a great book:
<Core C++: A Software Engineering Approach>

it's discuss deeply about function() and class and dynamic memory .etc..
it's very hard to comprehend for a newbie like me , but it's great!

whenever i see :: scope resolution operator in front of something with no left hand operand i think it's referrencing a globally defined object

In that case, the :: is used to indicate that you are calling a Win32 API function. This is sometimes necessary because MFC functions and Win32 functions will have the same name.

For instance, GetWindowText() will call the MFC function, but ::GetWindowText() will call the Win32 API function.

The :: operator can be used to explicitly access a globally defined object (in the 'unnamed' namespace)

For example, this program will output 3:

Code:

---

int x = 0;
int main()
{
  int x = 3; //local x hides the global x
  ::x = 4; //access global x explicitly
  cout x;
}

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