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Passing options to a function...

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Started by wasted_druid March 31, 2005 02:41 PM

6 comments, last by DigitalDelusion 19 years ago

wasted_druid

692

Author

March 31, 2005 02:41 PM

I see functions where you pass you options seperated by a '|' character... such as : SDL_Init(SDL_INIT_AUDIO|SDL_INIT_VIDEO|SDL_INIT_TIMER) My question is how I would code that myself. I'm assuming they're bit operators or some such... SDL_INIT_AUDIO = 1 SDL_INIT_VIDEO = 2 SDL_INIT_TIMER = 4 or some such... In other words, what exactly is happening in the above statement? Thanks.

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DigitalDelusion

874

March 31, 2005 02:45 PM

yup you got the right idea '|' in C and C++ is the bitwise or operator so the values are simple ored togheter and sent to the function.

The callee can then check the options using '&' (binary and) like so

if( options & SDL_INIT_AUDIO)//do audio init

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Omnibus

148

March 31, 2005 02:47 PM

The | is a bitwise OR operator. It is being used here to concatenate bits together. If you have an 8 bit number for the flags, you could set them with OR. For example 00000000 | 00000010 | 00100000 = 00100010

Also, note that that whole things you are passing to the function is just one argument that you are building not three arguments.

Other bitwise operators include & for AND, ~ for NOT, and ^ for XOR. Not to be confused with boolean operators &&, ||, and !

[Edited by - Omnibus on March 31, 2005 3:47:30 PM]

SirLuthor

364

March 31, 2005 02:54 PM

As the others have explained this well, I'll not bore you with more of the same, just figured I'd mention that those are generally refered to as flags, not options [smile] You'll get blank looks, methinks, if you go around spouting about 'options' :D

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weasalmongler

205

March 31, 2005 03:08 PM

Just a little addition to the topic.

You can easily create these flags using the bit shift operator. Eg:

(1 << 0) == 0000001
(1 << 1) == 0000010
(1 << 2) == 0000100
(1 << 3) == 0001000

etc... I just find it a bit easier than writing "#define FLAG1 2", "#define flag16 65536".

wasted_druid

692

Author

March 31, 2005 03:39 PM

Quote:Original post by SirLuthor
As the others have explained this well, I'll not bore you with more of the same, just figured I'd mention that those are generally refered to as flags, not options [smile] You'll get blank looks, methinks, if you go around spouting about 'options' :D

Thankyou. I hated asking the question since I didn't know the terminology. Heck, if I'd known the right term, I would have googled for it.

Anyways, thanks again.

smart_idiot

1,298

March 31, 2005 04:25 PM

Since nobody has said this yet, you can set a flag with flags |= FLAGN;, test if a flag is set, something like if(flags & FLAGN) {...}, and unset a flag with something like flags &= ~FLAGN;.

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DigitalDelusion

874

March 31, 2005 04:36 PM

and flip a flag with flags ^= FLAGN;

HardDrop - hard link shell extension."Tread softly because you tread on my dreams" - Yeats