Combining Arguments with the OR(|) Operator

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In the Win32 API(as well as in other API's), When passing arguments to some functions, I have seen and used the OR (|) to combine multiple parameters as one.

An Example:

C++

CreateWindow(...,
	     WS_VISIBLE | WS_CHILD | WS_DISABLED
             ,...);

In the above function call it combines WS_VISIBLE, WS_CHILD, WS_DISABLED into one value. Calling a function like that is fine.

Problem:
How does the CreateWindow() function catch those flags separately ? (I would like to use this method in my own projects)

Posted 28-May-13 20:09pm

Captain Price

Updated 28-May-13 20:10pm

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Sergey Alexandrovich Kryukov · Accepted Answer · 2013-05-28T20:23:00

This is done with the bit masks. Look at the values of WS_VISIBLE, WS_CHILD, etc.: http://msdn.microsoft.com/en-us/library/windows/desktop/ms632600%28v=vs.85%29.aspx[^].

As you can see, each of them is the order of 2. That is, each of them takes exactly one bit in the word. And you can always check a bit in one statement. Consider:

C++

BOOL isVisible = (myWindowStyle & WS_VISIBLE) > 0;
BOOL isChild = (myWindowStyle & WS_CHILD) > 0;
// ...

Bit-masking techniques could be much more complex than that, yet simple enough and very effective.

See also: http://www.cprogramming.com/tutorial/bitwise_operators.html[^].

—SA