C++ Macro Arguments w/ Token Concatenation? - c++

Question

I have a bunch of labeled servos, each one has its own calibrated min, mid and max pulse-width value.
// repository of calibrated servo pulse width values:
#define SERVO_0x01_MIN 165
#define SERVO_0x01_MID 347
#define SERVO_0x01_MAX 550
#define SERVO_0x02_MIN 165
#define SERVO_0x02_MID 347
#define SERVO_0x02_MAX 550
...
To simplify maintenance of the code, swapping a servo should only require changing a single macro definition value.
// maps certain positions on robot to the servo that is installed there
#define JOINT_0 0x02
#define JOINT_1 0x05
#define JOINT_2 0x0A
...
// function-like macros to resolve values from mapping
#define GET_MIN(servo) SERVO_##servo##_MIN
#define GET_MID(servo) SERVO_##servo##_MID
#define GET_MAX(servo) SERVO_##servo##_MAX
The problem I'm having is that calling a function-like macro with an argument that itself is a macro does not resolve to its terminal value:
// main
int main(void) {
// this works
int max_0x01 = GET_MAX(0x01); // int max_0x01 = 550;
// this doesn't
int max_joint_0 = GET_MAX(JOINT_0); // int max_joint_0 = SERVO_JOINT_0_MAX;
}
What can I do to make GET_MAX(JOINT_0) turn into 550 ?

Answer 1

#define GET_MAX(servo) GET_MAX2(servo)
#define GET_MAX2(servo) SERVO_##servo##_MAX
The preprocessor will perform expansion (textual replacement) upon a variadic macro until it can expand no further. So passing in JOINT_0, such as GET_MAX(JOINT_0) will expand to
GET_MAX2(0x02)
This gets further expanded to
SERVO_0x02_MAX
And finally replaced with the #define value 550