error LNK2019: "unresolved external symbol" using variadic function

Question

Trying to build function that gets unknown number of arguments.

Header.h:

void WriteHeaderToCSVFile() {}

template <typename FieldType, typename... Types>
void WriteHeaderToCSVFile(FieldType field1, Types...field2);

Source.cpp:

template <typename FieldType, typename... Types>
void CSVWriter::WriteHeaderToCSVFile(FieldType field1, Types... field2)
{
    mFileStream->Write(field1);
    mFileStream->Write(scComma);
    WriteHeaderToCSVFile(field2...);
}

void main()
{
   myFile2.WriteHeaderToCSVFile(L("no.record"), L("no.page"), L("values"));
}

Getting the error bellow:

error LNK2019: unresolved external symbol "public: void __cdecl CSVWriter::WriteHeaderToCSVFile<wchar_t const *,wchar_t const *,wchar_t const *>(wchar_t const *,wchar_t const *,wchar_t const *)" (??$WriteHeaderToCSVFile@PEB_WPEB_WPEB_W@XMPCSVWriter@@QEAAXPEB_W00@Z) referenced in function "void __cdecl TestCSVFileWriter(void)" (?TestCSVFileWriter@@YAXXZ)

score 0 · Answer 1 · answered Feb 13 '22 at 19:53

I suppose the problem is that you need an "empty" overload for WriteHeaderToCSVFile() as ground case for the recursion; I mean

void CSVWriter::WriteHeaderToCSVFile ()
{ }

I mean... myFile2.WriteHeaderToCSVFile(L("no.record"), L("no.page"), L("values")); call myFile2.WriteHeaderToCSVFile(L("no.page"), L("values")); that matches the variadic method (L("no.page") matches field1, L("values") matches field2...)

Then myFile2.WriteHeaderToCSVFile(L("no.page"), L("values")); call myFile2.WriteHeaderToCSVFile(L("values")); that matches the variadic method (L("values") matches field1, field2... matches as empty).

The myFile2.WriteHeaderToCSVFile(L("values")); call myFile2.WriteHeaderToCSVFile(); but nothing matches field1, for the variadic method.

So you need another WriteHeaderToCSVFile() that can match an empty list of argument.

Off Topic Unrequested Suggestion: define your templates (classes, functions, methods, variables, etc.) directly in the header files where they are declared.

The reason is well described in here

Thank you Tried that and now the error is: error LNK2019: unresolved external symbol "public: void __cdecl CSVWriter::WriteHeaderToCSVFile(int,int,int)" Change the call in main to: myFile2.WriteHeaderToCSVFile(1,2,3); — טל היינה, Feb 14 '22 at 11:47
@טלהיינה - Sorry but, without a complete example (possibly minimal), to reproduce the error, is very difficult to say where the problem is. — max66, Feb 14 '22 at 13:09

error LNK2019: "unresolved external symbol" using variadic function

1 Answers1