Syntax of binary Constants

Question

I was trying to compile some Arduino code on the PC for testing when I noticed some strange syntax for numerical binary constants.

Convetion seems to be to declare them like so:

    static const uint8_t smiley[] = {
        B00111100,
        B01000010,
        B10100101,
        B10000001,
        B10100101,
        B10011001,
        B01000010,
        B00111100 };

However, when I try to compile this with g++ I get

error: ‘B00111100’ was not declared in this scope

Now when I exchange this for B for the standard C/C++ prefix 0B it works, but I don't want to change the code that I want so simulate!

Is there some clever macro magic at play here?

score 4 · Accepted Answer · answered Dec 10 '18 at 23:32

There is a file (binary.h) with every possible permutation of number between 0 and 255 represented as binary in it. They're stored as C-preprocessor macros:

#define B0 0
#define B00 0
#define B000 0
#define B0000 0
#define B00000 0
#define B000000 0
#define B0000000 0
#define B00000000 0
#define B1 1
#define B01 1
#define B001 1
#define B0001 1
#define B00001 1
#define B000001 1
#define B0000001 1
#define B00000001 1
#define B10 2
#define B010 2
#define B0010 2
#define B00010 2
#define B000010 2
#define B0000010 2
#define B00000010 2
... etc ...

Quite why Arduino decided to do this is anyone's guess. It makes, as you have seen, for non-portable code.

Personally, I never use (and would never recommend any one else should use) those macros, and instead use the GCC standard binary literal representation 0b.....

Syntax of binary Constants

1 Answers1