incrementally create arrays at compile time

Question

Inspired by this post https://stackoverflow.com/a/6730362 I'm trying to set up a compile-time Sequence of similar (integral) types that may be stored in ROM only using template metaprogramming. The idea is to define an array incrementally by adding elements line by line using the variadic scheme of template metaprogramming

Currently I'm here:

template<typename T, T... values> struct Seq
{
    static const T value[sizeof...(values)];
    static const uint16 n;
};
template<typename T, T... values> const T       Seq<T, values...>::value[]  = { values };
template<typename T, T... values> const uint16  Seq<T, values...>::n        = sizeof...(values);

template<sint32 idx, typename T, T... values> struct IncSeq;
template<sint32 idx, typename T, T... values> struct IncSeq<-1, T, values...> : Seq<T, values...> {};

I'm fairly new to template metaprogramming. So I basically have no clue what I'm doing here. But the VS-compiler complains about some partial specialization issues:

'idx' : template parameter not used or deducible in partial specialization 'IncSeq<-1,T,values...>'

Can you help me out here? Why isn't it deducible? I wrote '-1'!

the ovedrall idea is to have index variable decrement during each call until -1 is reached which causes the IncSeq to inherit from Seq using the provided types. Usage like so:

template<sint32 idx, typename T, T... values> struct IncSeq<0, uint8, values...> : IncSeq<-1, uint8, 42, values..>{};
template<sint32 idx, typename T, T... values> struct IncSeq<1, uint8, values...> : IncSeq<0, uint8, 43, values..>{};
template<sint32 idx, typename T, T... values> struct IncSeq<2, uint8, values...> : IncSeq<1, uint8, 44, values..>{};
typedef IncSeq<2, uint8> myStaticArray;

I realize this looks like a lot of hazzle but the idea of having an array in ROM being constructed line by line in source code may pose some advantages to some of us.

score 0 · Accepted Answer · answered Mar 27 '17 at 14:47

Can you help me out here? Why isn't it deducible? I wrote '-1'!

The compiler cannot deduce idx exactly because you wrote -1. Since you're explicitly specifying what idx should be, you should get rid of it from the template parameters:

template<typename T, T... values> struct IncSeq<-1, T, values...> : Seq<T, values...> {};

You also need to expand values here:

template<typename T, T... values>
const T Seq<T, values...>::value[] = { values... };

Your code will now compile. (I have not tested the correctness of your solution.)

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incrementally create arrays at compile time

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