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How to use formatting flags?

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Started by Aternus June 12, 2006 12:18 AM

2 comments, last by Aternus 17 years, 10 months ago

Aternus

121

Author

June 12, 2006 12:18 AM

By formatting flags I mean using the | bitwise operator to input several options to a function like C++ file I/O where you use something like file.open("f.txt",ios::in|ios::ate|ios::trunc). I'm wondering how I could use these myself. I was experimenting with it quite a bit and don't really see how it's possible. Like unsigned int mode = (2|3|7); gets you 7... And unsigned int mode = (2|7); also gets you 7. So I don't really see how, in any way bitwise operators could really seperate them for you after being OR'd. Could anyone please shed some light on this? Thanks. :) -Aternus

Boder

935

June 12, 2006 12:26 AM

Your example doesn't work because 2 and 3 share a bit. For this method to work each option must be a power of two.

Each bit represents a certain option.

0000000000000001 - thrusters on0000000000000010 - shield on0000000000000100 - boost on

In hex:

#define THRUSTER_ON 0x0001#define SHIELD_ON   0x0002#define BOOST_ON    0x0004

To use:

activate( THRUSTER_ON | SHIELD_ON );// passes 16-bit integer 0000000000000011 which has the options set

Therefore, for a 16-bit number you can have a maximum of 16 options, because each bit has to be mutually exclusive. This is opposed to using normal enumerations where each separate number gives you an option, so a 16-bit integer can hold 2^16 values.

Cocalus

554

June 12, 2006 12:28 AM

You need to use powers of 2 for your numbers.

Maybe an example program will help.

#include <iostream>using namespace std;const unsigned int bit0 = 1;const unsigned int bit1 = 2;const unsigned int bit2 = 4;const unsigned int bit3 = 8;const unsigned int bit4 = 16;const unsigned int bit5 = 32;const unsigned int bit6 = 64;const unsigned int bit7 = 128;//etcvoid printbits(const unsigned int flags){     if (flags&bit0)        cout << "bit0 ";     if (flags&bit1)        cout << "bit1 ";     if (flags&bit2)        cout << "bit2 ";     if (flags&bit3)        cout << "bit3 ";     if (flags&bit4)        cout << "bit4 ";     if (flags&bit5)        cout << "bit5 ";     if (flags&bit6)        cout << "bit6 ";     if (flags&bit7)        cout << "bit7 ";     cout << endl;}int main(int argc, char *argv[]){        printbits(bit6|bit3|bit1);    return 0;}