How to declare defines with exponential

Question

I'm trying to create the following defines, my source code is shared between an iOS app and a C++ ARM firmware.

#define ASSIGN_MIN_VAL_NB_BITS      7
#define ASSIGN_MIN_VAL_BIT_POS      1
#define ASSIGN_MIN_VAL_BIT_MASK     (((2^ASSIGN_MIN_VAL_NB_BITS)-1)<<ASSIGN_MIN_VAL_BIT_POS)

I'm expecting ASSIGN_MIN_VAL_BIT_MASK to be 0b11111110, but it is not. The above 2^ seems to be the problem. How could I declare something similar ? I've tried using pow(x,y) to replace the 2^, it works but I would like to find a way to declare these define without using runtime functions (I assume pow is a runtime function).

Any idea, tip greatly appreciated.

Answer 1

1. Don't use #define for constants in C++, use const variables.

2. ^ is bitwise XOR, not exponentiation. 2 to the power of x can be represented as 1 << x.

So in your case, the correct expression would be:

((1 << ASSIGN_MIN_VAL_NB_BITS) - 1) << ASSIGN_MIN_VAL_BIT_POS