Or'ing function parameters c++

Question

I have noticed that many functions take parameters or "flags" like so:

foo(BIG | RED | SWEET);

Where BIG, RED, and SWEET have been #define'ed earlier in the file such as: #define BIG 0x1

I want to implement my own functions that take parameters like above but I am worried that ORing two numbers together might equal the same result as ORing two different numbers. What is the proper way to define these variables so there is no collision?

These are bitwise ORs. So the variables are usually zero and powers of 2. 0, 1, 2, 4... — juanchopanza, Jun 04 '14 at 21:40

score 0 · Answer 1 · answered Jun 04 '14 at 21:42

0

Usually the flags will be powers of 2, ensuring that each combination is unique.

answered Jun 04 '14 at 21:42

mc110

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bstar55 · Answer 2 · 2014-06-04T23:38:28.663

0

The following powers of 2 hex pattern is commonly used to avoid collisions with flags:

#define A    (0x0000)
#define B    (0x0001)
#define C    (0x0002)
#define D    (0x0004)
#define E    (0x0008)
#define F    (0x0010)
#define G    (0x0020)
#define H    (0x0040)
#define I    (0x0080)
#define J    (0x0100)
...

Or better yet, use enums:

enum Flags
{
    A = 0,
    B = 1 << 0,
    C = 1 << 1,
    D = 1 << 2,
    E = 1 << 3,
    F = 1 << 4,
    ...
}

This C++ approach far is more pleasing to the eyes than those hex defines

edited Jun 04 '14 at 23:38

answered Jun 04 '14 at 21:43

bstar55

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You certainly should advise for usage of `enum` instead of a bunch of `#define` directives! – πάντα ῥεῖ Jun 04 '14 at 21:45
Ha! Good grief! Updated. – bstar55 Jun 04 '14 at 21:52
`0x0000` is not a power of two. It is not possible to recover your A flag from a resulting bitset. – Red Alert Jun 05 '14 at 00:23
That's why 0 traditionally refers to no flags set. – bstar55 Jun 05 '14 at 00:28
@bstar55 in that case you could just use `if(!flags)`, instead of the more convoluted `if(flags|A==A)` or `if(flags==A)` – Red Alert Jun 05 '14 at 02:19
Yep, your absolutely correct. – bstar55 Jun 05 '14 at 02:43

Or'ing function parameters c++

2 Answers2