How do you use bitwise flags in C++?

Question

As per this website, I wish to represent a Maze with a 2 dimensional array of 16 bit integers.

Each 16 bit integer needs to hold the following information:

Here's one way to do it (this is by no means the only way): a 12x16 maze grid can be represented as an array m[16][12] of 16-bit integers. Each array element would contains all the information for a single corresponding cell in the grid, with the integer bits mapped like this:

_{(source: mazeworks.com)}

To knock down a wall, set a border, or create a particular path, all we need to do is flip bits in one or two array elements.

How do I use bitwise flags on 16 bit integers so I can set each one of those bits and check if they are set.

I'd like to do it in an easily readable way (ie, Border.W, Border.E, Walls.N, etc).

How is this generally done in C++? Do I use hexidecimal to represent each one (ie, Walls.N = 0x02, Walls.E = 0x04, etc)? Should I use an enum?

See also How do you set, clear, and toggle a single bit?.

score 10 · Answer 1 · answered Feb 13 '09 at 18:34

10

If you want to use bitfields then this is an easy way:

typedef struct MAZENODE
{
    bool backtrack_north:1;
    bool backtrack_south:1;
    bool backtrack_east:1;
    bool backtrack_west:1;
    bool solution_north:1;
    bool solution_south:1;
    bool solution_east:1;
    bool solution_west:1;
    bool maze_north:1;
    bool maze_south:1;
    bool maze_east:1;
    bool maze_west:1;
    bool walls_north:1;
    bool walls_south:1;
    bool walls_east:1;
    bool walls_west:1;
};

Then your code can just test each one for true or false.

answered Feb 13 '09 at 18:34

KPexEA

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unfortunately this is not portable as the order of bits is compiler specific. – Evan Teran Feb 13 '09 at 18:38
Also, I'm not sure if bool is a supported type for bitfields, it would be more portable if unsigned were used. – Evan Teran Feb 13 '09 at 18:38
yes bool is supported for bitfields, but you are correct, ordering of them is totally up to the compiler and usually associated with endianess – KPexEA Feb 13 '09 at 18:56
3

Unless you want to send this binary data structure between machines, or store it to a file and read it back with another program, the order of bits is unimportant. So this gets +1 vote from me. – KeyserSoze Feb 13 '09 at 21:39
Would be even nicer if you could nest the n,s,e,w fields into an internal struct. – xtofl Mar 20 '09 at 12:14

score 8 · Accepted Answer · edited Apr 26 '13 at 19:54

8

Use std::bitset

edited Apr 26 '13 at 19:54

answered Feb 14 '09 at 09:12

Vinay

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score 3 · Answer 3 · answered Feb 13 '09 at 18:13

3

Use hex constants/enums and bitwise operations if you care about which particular bits mean what.

Otherwise, use C++ bitfields (but be aware that the ordering of bits in the integer will be compiler-dependent).

answered Feb 13 '09 at 18:13

moonshadow

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score 2 · Answer 4 · answered Feb 13 '09 at 18:17

Learn your bitwise opertors: &, |, ^, and !.

At the top of a lot of C/C++ files I have seen flags defined in hex to mask each bit.

#define ONE 0x0001

To see if a bit is turned on, you AND it with 1. To turn it on, you OR it with 1. To toggle like a switch, XOR it with 1.

score 2 · Answer 5 · answered Mar 20 '09 at 12:16

2

To manipulate sets of bits, you can also use ....

std::bitset<N>

std::bitset<4*4> bits;
bits[ 10 ] = false;
bits.set(10);
bits.flip();
assert( !bits.test(10) );

answered Mar 20 '09 at 12:16

xtofl

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score 1 · Answer 6 · answered Feb 13 '09 at 18:10

1

You can do it with hexadecimal flags or enums as you suggested, but the most readable/self-documenting is probably to use what are called "bitfields" (for details, Google for C++ bitfields).

answered Feb 13 '09 at 18:10

ChrisW

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score 1 · Answer 7 · answered Feb 13 '09 at 18:15

Yes a good way is to use hex decimal to represent the bit patterns. Then you use the bitwise operators to manipulate your 16-bit ints.

For example:

if(x & 0x01){} // tests if bit 0 is set using bitwise AND
x ^= 0x02;     // toggles bit 1 (0 based) using bitwise XOR
x |= 0x10;     // sets bit 4 (0 based) using bitwise OR

score 0 · Answer 8 · answered May 19 '09 at 04:12

I'm not a huge fan of bitset. It's just more typing in my opinion. And it doesn't hide what you are doing anyway. You still have to & && | bits. Unless you are picking on just 1 bit. That may work for small groups of flags. Not that we need to hide what we are doing either. But the intention of a class is usually to make something easier for it's users. I don't think this class accomplishes it.

Say for instance, you have a flag system with .. 64 flags. If you want to test.. I don't know.. 39 of them in 1 if statement to see if they are all on... using bitfields is a huge pain. You have to type them all out.. Course. I'm making the assumption you use only bitfields functionality and not mix and match methods. Same thing with bitset. Unless I am missing something with the class.. which is quite possible since I rarely use it.. I don't see a way you can test all 39 flags unless you type out the hole thing or resort to "standard methods" (using enum flag lists or some defined value for 39 bits and using the bitsets && operator). This can start to get messy depending on your approach. And I know.. 64 flags sounds like a lot. And well. It is.. depending on what you are doing. Personally speaking, most of the projects I'm involved with depend on flag systems. So actually.. 64 is not that unheard of. Though 16~32 is far more common in my experience. I'm actually helping out in a project right now where one flag system has 640 bits. It's basically a privilege system. So it makes some sense to arrange them all together... However.. admittedly.. I would like to break that up a bit.. but.. eh... I'm helping.. not creating.

How do you use bitwise flags in C++?

8 Answers8