Some bug in template with enum parameter

[CakeByTheOcean]

Hey. I want to write a helper function `count` to give me a int back, basing on the template parameter enum, which tell which int(8, 16, 32, 64) the pointer is pointing to and how to compare the predicate with the stored data. I am not sure about how to write such a template helper function with enum parameter, which I think not belong to a class.


I got some bug with having a template function with enum parameter.
(^^ is the postion the error comes)

--------for DataBlock.hh--------------------------
error: use of ‘this’ in a constant expression
return count<encodingType^^, predicate.compareType>(predicate.val, getDataLength(predicate.col_id), basePointer);


error: no matching function for call to ‘count<((DataBlock*)this)->DataBlock::encoding_type.std::array<imlab::EncodingType, 3>::operator[](((std::array<:EncodingType, 100>::size_type)predicate.Predicate::col_id)), predicate.Predicate::compareType>(uint64_t&, uint32_t, unsigned char*&)’
return count<encoding_types[predicate.col_id], predicate.compareType>^^(predicate.val, getDataLength(predicate.col_id), basePointer);

--------for util.hh--------------------------
note: candidate: template<EncodingType E, CompareType C> uint64_t count(uint64_t, uint32_t, uint8_t*)
uint64_t count(uint64_t val, uint32_t dataLength, uint8_t* basePointer);
^^~~~~

note: template argument deduction/substitution failed:
error: ‘this’ is not a constant expression
count<encoding_types[predicate.col_id], predicate.compareType>^^(predicate.val, getDataLength(predicate.col_id), basePointer);

note: in template argument for type ‘EncodingType’
error: ‘predicate’ is not a constant expression
note: in template argument for type ‘CompareType’

//----------base enum and struct---------------
enum EncodingType {
   byte1,
   byte2,
   byte4,
   byte8
};

enum CompareType {
  EQUAL,
  GREATER_THAN,
  LESS_THAN
};

struct Predicate {
  uint32_t col_id;
  uint64_t val;
  CompareType compareType;
};

//---------------------util.hh----------------------------
//---------template function with enum-----------

template <EncodingType E, CompareType C>
uint64_t count(uint64_t val, uint32_t dataLength, uint8_t* basePointer);

template <>
uint64_t count<EncodingType::byte1, CompareType::EQUAL>(uint64_t val, uint32_t dataLength, uint8_t* basePointer) {
  uint64_t counter = 0;
  val = (uint8_t) val; 
  for(size_t in = 0 ; in < dataLength; in++) {
    if (*(basePointer + in) == val) {
      ++counter;
    }
  }
  return counter;
}
// other combinations are simliar


//------------DataBlock.hh--------------
class DataBlock{
 // some attributs
 std::array<EncodingType, 100> encoding_types;

 // a member funciton in DataBlock class
 uint64_t scan(Predicate& predicate) {
  return count<encoding_types[predicate.col_id], predicate.compareType>(predicate.val, getDataLength(predicate.col_id), basePointer);
// assumed here we get the length via getDataLength(predicate.col_id) and the basePointer is correct
}
}

[jonnin]

misread it. looking again.

[CakeByTheOcean]

I come up with my solution.
Hmm I think we have 5 * 3 situations. (5 for encoding and 3 for the compare)
Then just make `switch case` on these 15 situations.
It would work but not so beautiful.
--------
We would also try to discuss it to come up with a more elegant version.