How to store indicies of an array in preprocessor defines? - c++

Question

I am creating a VST(virtual instrument) program in cpp and I have an array of structs that represent various parameters in my program:
const FloatParam_Properties FloatParamProps[NUM_FLOAT_PARAMS] =
{
//Frequency
{"BaseFreq", "Base Freq", 0.0, 20.0, 5.0, 0.6}, //0
{"FreqDelta", "Freq Delta", -20.0, 20.0, 0.0, 0.6}, //1
...
//Wave
{"OscSelect", "Wave form", 0.0, 3.0, 0.0, 1.0}, //9
//Master
{"Volume", "Volume", 0.0, 1.0, 0.1, 0.4}, //10
};
Each element in this array is a struct, when I want to access these structs I have just hardcoded the indices(e.g. doing FloatParamProps[0] to access the Base Frequency). However, I would like to use the cpp preprocessor to give these hard-coded indices a name since everything here is known at compile-time and it would be good if I didn't have to just hard-code those defines either.
What I would like to do is something like:
const FloatParam_Properties FloatParamProps[NUM_FLOAT_PARAMS] =
{
//Frequency
DEF_FLOAT_PARAM(BaseFreq, "Base Freq", 0.0, 20.0, 5.0, 0.6), //0
DEF_FLOAT_PARAM(FreqDelta, "Freq Delta", -20.0, 20.0, 0.0, 0.6), //1
...
//Wave
DEF_FLOAT_PARAM(OscSelect, "Wave form", 0.0, 3.0, 0.0, 1.0), //9
//Master
DEF_FLOAT_PARAM(Volume, "Volume", 0.0, 1.0, 0.1, 0.4), //10
};
Where DEF_FLOAT_PARAM was a macro that would then take the first argument and turn that into a pre-processor define(or maybe constexpr) by using the __COUNTER__ macro. Then, if I wanted to access the first one I could do FloatParamProps[BaseFreq], for example. The issue I'm having is that you can't have a define within a macro so I can't define BaseFreq as a constant.
I also tried doing something like
#define DEF_FLOAT_PARAM(ID, Name, minVal, maxVal, defaultVal, skewFactor) P_##ID __COUNTER__ \
{#ID, Name, minVal, maxVal, defaultVal, skewFactor},
and the plan here was to try and take the define outside of the macro and just type that manually like so:
const FloatParam_Properties FloatParamProps[NUM_FLOAT_PARAMS] =
{
//Frequency
#define DEF_FLOAT_PARAM(BaseFreq, "Base Freq", 0.0, 20.0, 5.0, 0.6), //0
...
But the issue with that is that the pre-processor doesn't want to expand that macro when it's in-front of the define. If only I could tell it to expand the macro, it would work.
Does anyone know how I could do this?
Thanks.

Answer 1

Use x-macros:
#define PARAMS(X) \
X(BaseFreq, "Base Freq", 0.0, 20.0, 5.0, 0.6) \
X(FreqDelta, "Freq Delta", -20.0, 20.0, 0.0, 0.6) \
X(OscSelect, "Wave form", 0.0, 3.0, 0.0, 1.0)
const FloatParam_Properties FloatParamProps[NUM_FLOAT_PARAMS] =
{
#define PARAM_STRUCT(id, ...) {#id, __VA_ARGS__},
PARAMS(PARAM_STRUCT)
#undef PARAM_STRUCT
};
enum Params
{
#define PARAM_ENUM(id, ...) id,
PARAMS(PARAM_ENUM)
#define PARAM_ENUM
_count,
};
This will generate the same array you already have, and a enum with the constants matching the array indices.