C++ Pass template argument through multiple functions

Question

Consider these lines of code. When I try to compile the compiler will show errors like 'a' is not a member of 'DataType1'. I understand how the compiler treats these as errors but is there any way I could avoid this or another method that works?

struct DataType1 { public: int x; };
struct DataType2 { public: int a; };

template <class E>
bool job2(E* newData, const int i){
  int something = 2;  
  if (i == 1) newData->x = something;
  if (i == 2) newData->a = something;
}

template <class E>
bool job1(List<E>* dataBase){
  E* newData = new E;
  job2(newData, 1);
  dataBase->push_back(newData);
}

template <class E>
int main(){
  List<DataType1>* dataBase = new List<DataType>;
  job1(dataBase);
}

You can turn the const int i variable into a template argument and specialize the templated function. But that brings a question: why don't you remove the i argument and just specialize on E? — freakish
– freakish, Commented Sep 23, 2019 at 9:13
Another way could be to define a set function with the same name (SetI()) in each DataType struct and call that instead newData->SetI(something). — nielsen
– nielsen, Commented Sep 23, 2019 at 9:16
If you can use C++17, and if you can pass i as template parameter, you can use if constexpr — max66
– max66, Commented Sep 23, 2019 at 9:16
@freakish I tried overloading with job2(DataType1* newData) but it couldn't compile with error cannot convert argument from E* to DataType1 — Hà Huy Long Hải
– Hà Huy Long Hải, Commented Sep 23, 2019 at 9:30
@HàHuyLongHải that's because you try to push_back an element of type E* to List<DataType1>. That's obviously wrong, even when E=DataType1. The problem is job1 function, not job2 specialization/overload. Not to mention that job1 leaks memory. — freakish
– freakish, Commented Sep 23, 2019 at 9:46

Timo · Accepted Answer · 2019-09-23 09:23:22Z

2

If you have C++17 at hand you can write:

template <class E>
bool job2(E* newData){
  int something = 2;  
  if constexpr (std::is_same_v<E, DataType1>) 
     newData->x = something;
  else 
     newData->a = something;
}

and discard i alltogether (if you only used it to distinguish between types).

Othwerwise, what argues against simply overloading your function?

bool job2(DataType1* newData){
  commonOperation();
  newData->x = something;
}

bool job2(DataType2* newData){
  commonOperation();
  newData->a = something;
}

where commonOperation is everything the functions have in common.

edited Sep 23, 2019 at 9:23

answered Sep 23, 2019 at 9:18

Timo

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1 Comment

Hà Huy Long Hải Over a year ago

I tried the second one but it couldn't compile with error cannot convert argument from E* to DataType1

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