Mapping elements of a structure to different vector elements using reflection (compile-time)

Question

I recently have hit a wall, and can't seem to find an elegant solution to my problem:

I have different data structures of different number of elements, all of which are aligned to pointer-size and are not larger than a pointer, ex.:

struct DATA_0{
    size_t      Value;
    size_t      *PtrValue;
    bool        BoolValue;
};

or

struct DATA_1{
    size_t      Value0;
    size_t      Value1;
    size_t      *PtrValue;
    void        *PtrBuffer;
    bool        BoolValue;
    const char  *StringBuffer;
}

The input comes in a form like this:

struct INPUT{
    size_t *DataBuffer;
    size_t DataVector[16];
};

I need to map the data structures to different elements from either the DataBuffer or the DataVector, at compile time:

the mapping rule is being controlled by #defines/compile time switches - for a given compilation configuration, the mapping rule is the same for all structures.
a given data structure member can be mapped to either a DataBuffer or a DataVector
a data structure member's index (no need for offset, since the data is aligned) defines the element it is mapped to, ex.: in some cases the element at index 0 (DATA_0::Value or DATA_1::Value0 for example) maps to DataVector[3], and in other cases to DataBuffer[0]
for those elements, that have no index based mapping rules (usually the first 2-4 elements from a structure have such rule), they all should map to sequential elements, onward from a given element from DataBuffer

Some example of desired usage:

INPUT_DATA *input;

data_mapping<DATA_0> mapping_0(input);
//will fetch data from input->DataVector[0] in some cases, input->DataBuffer[0] in other
size_t tmpValue = mapping_0[Value];

data_mapping<DATA_1> mapping_1(input);
//will fetch data from input->DataVector[1] in some cases, input->DataBuffer[3] in other
size_t tmpPtrBuffer = mapping_1[PtrBuffer];

NOTE: the data does not need to be necessarily represented by structures, it can be even variadic template argument too, ex.: <size_t, Value, size_t, PtrValue, bool, BoolValue>

Here is an pseudo-code example of what i am trying to achieve:

template<class TMember, class TData>
TMember GetElement(INPUT_DATA *Input, TData Data)
{
#ifdef FIRST_CASE
    if(indexof(TMember) == 0)
    {
        return Input->DataVector[0];
    }
    if(indexof(TMember) == 1)
    {
        return Input->DataVector[1];
    }
    return Input->DataBuffer[indexof(TMember) - 2];
#else
    if(indexof(TMember) == 0)
    {
        return Input->DataVector[2];
    }
    if(indexof(TMember) == 1)
    {
        return Input->DataVector[3];
    }
    if(indexof(TMember) == 2)
    {
        return Input->DataVector[4];
    }
    if(indexof(TMember) == 3)
    {
        return Input->DataVector[5];
    }
    return Input->DataBuffer[indexof(TMember)];
#endif
}

size_t tmpValue = GetElement<Value>(input,DATA_0);
size_t tmpPtrBuffer = GetElement<PtrBuffer>(input,DATA_1);

Caleth · Accepted Answer · 2024-08-09 09:39:47Z

1

Value and PtrBuffer are not valid names on their own. &DATA_0::PtrBuffer and &DATA_1::PtrBuffer are, but they have different types, so you'll have a whole load of overloads of operator[] for your desired syntax.

I think the easiest thing for you to do here is just write out some functions

DATA_0 to_data_0(INPUT input) {
    DATA_0 result;
    result.Value = input.DataVector[0];
    // etc.
    return result;
}

DATA_1 to_data_1(INPUT input) {
    DATA_1 result;
    result.Value0 = input.DataVector[2];
    // etc.
    return result;
}

If you have different configurations, then you will need different versions of these functions.

answered Aug 9, 2024 at 9:39

Caleth

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user2281752 · Accepted Answer · 2025-04-09 19:08:01Z

I ended up using some (finitely) recursing macros to generate class types, that has named functions, that map to different data fields, depending on compile-time macros:

template<class TBase> class DataMapping : public TBase {
protected:
    template<class T> inline const T &GetData(size_t Index) const{
#ifdef FIRST_CASE
        return (T &)(this->GetDataBuffer()[Index]);
#elif SECOND_CASE
        if (0 == Index) { return (T &)(this->DataVector()->data_0); }
        if (1 == Index) { return (T &)(this->DataVector()->data_1); }
        if (2 == Index) { return (T &)(this->DataVector()->data_2); }
        return (T &)(this->GetDataBuffer()[Index]);
#elif THIRD_CASE
        if (0 == Index) { return (T &)(this->DataVector()->data_0); }
        if (1 == Index) { return (T &)(this->DataVector()->data_1); }
        return (T &)(this->GetDataBuffer()[Index]);
#endif
    }
};

#define GEN_GETTER(_Type, _Name, _Index) \
    typedef _Type FIELD_##_Name##_TYPE; \
    inline const FIELD_##_Name##_TYPE &Data_##_Name() const { \
        return GetData<_Type>(_Index); \
    } \
    inline FIELD_##_Name##_TYPE & Data_##_Name() { \
        return const_cast<_Type &>(const_cast<decltype(this)>(this)->GetData<_Type>(_Index)); \
    }

#define EXPAND(_expr) _expr

#define GEN_GETTER_0(...)
#define GEN_GETTER_1(_Index, _Type, _Name) GEN_GETTER(_Type, _Name, _Index)
#define GEN_GETTER_2(_Index, _Type, _Name, ...)  GEN_GETTER(_Type, _Name, _Index) EXPAND(GEN_GETTER_1(_Index + 1, __VA_ARGS__))
...
#define GEN_GETTER_31(_Index, _Type, _Name, ...) GEN_GETTER(_Type, _Name, _Index) EXPAND(GEN_GETTER_30(_Index + 1, __VA_ARGS__))
#define GEN_GETTER_32(_Index, _Type, _Name, ...) GEN_GETTER(_Type, _Name, _Index) EXPAND(GEN_GETTER_31(_Index + 1, __VA_ARGS__))
#define GEN_GETTER_N(_NData, ...) EXPAND(GEN_GETTER_##_NData(0, __VA_ARGS__))


#define DECLARE_DATA_MAP(_ClassName, _BaseClass, _NData, ...) \
struct _ClassName : public DataMapping<_BaseClass> { \
    GEN_GETTER_N(_NData, __VA_ARGS__) \
};

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