Thread: Unresolved external with template function

I am trying to use a template function to overload the >> stream operator but keep getting unresolved external errors (VC++ 6.0).

The function definition and declaration are:

Code:

// .h file **********************
#include <fstream>

using namespace std;

class BinStreamIn : public ifstream 
{
   
public:
   BinStreamIn( const char *inputFileName );
   void ReadBytes(void *inputBytes, int numOfBytesToInput);
   template <class T> BinStreamIn & operator>> (T &inputValue);
 .
 .
 .
}   

//.cpp file ***********************
template <class T> BinStreamIn & BinStreamIn::operator>> (T &inputValue) 
{
   BinStreamIn::ReadBytes( &inputValue, sizeof(inputValue) );
  return *this;

} // end function BinStreamIn::operator>>

--------------------------
Linking...
DataStreamReader.obj : error LNK2001: unresolved external symbol "public: class BinStreamIn & __thiscall BinStreamIn::operator>>(unsigned short &)" (??5BinStreamIn@@QAEAAV0@AAG@Z)
Release/FEP.exe : fatal error LNK1120: 1 unresolved externals
Error executing link.exe.

Any ideas?

There is other code in the cpp file so I need to keep it. Why does the templated function need to be in the h file though? Doesn't a #include get you what you need?

I put the template function in the .h file and it does compile, but I now get the following error in the function that calls it:

Code:

DataStreamReader.cpp
C:\ABI Workspace\MPI\MPI_Split_exes\FEP\DataStreamReader.cpp(29) : error C2679: binary '>>' : no operator defined which takes a right-hand operand of type 'unsigned short' (or there is no acceptable conversion)

Now it seems to think I'm trying to use the binary shift operator instead of my overloaded extraction operator.

Here is the function... (VALUE_TYPE is typedef'd as an unsigned short int)

Code:

#include "DataStreamReader.h"

int DataStreamReader::GetValues(BinStreamIn dataFile, VALUE_TYPE values[]) 
{
   // Local Variables
   unsigned int counter = 0;
   
//   while((counter < MAX_VALS) && (dataFile >> values[counter]))
   while(counter < MAX_VALS)
   {
      dataFile >> values[counter];  //Here's the use of the >> operator 
      counter++;
      cout << "#vals = " << counter - 1 << "  value read = " << (VALUE_TYPE)values[counter - 1] << endl;
   }
      
   return counter;
   
} // end function GetValues

Did you remember to #include the header that contains BinStreamIn? The code you posted compiles fine for me with some added stuff around it. You'll probably want to post more code to see the problem.

The reason you need to put all the code for a templated class into the header file is that the code is just a template and the compiler needs to compile the code with each type that will be used with it. If you add it to the header file included by files that use it, then the compiler will see which types are used with it and compile them properly.

BTW, I don't think you should be passing the stream by value. You probably meant to make that a reference argument.

You are right about passing by reference. That's the way I had it initially but tried passing by value when I couldn't find any fix for this problem. I forgot to change it back.

I do have a #include to BinStreamIn.h in DataStreamReader.h

Code:

#include "BinStreamIn.h"

class DataStreamReader
{
public:
   int GetValues(BinStreamIn &dataFile, VALUE_TYPE values[]);
};

Code:

#include "DataStreamReader.h"

int DataStreamReader::GetValues(BinStreamIn &dataFile, VALUE_TYPE values[]) 
{
   // Local Variables
   unsigned int counter = 0;
   
   while(counter < MAX_VALS)
   {
      dataFile >> values[counter];
      counter++;
      cout << "#vals = " << counter - 1 << "  value read = " << (VALUE_TYPE)values[counter - 1] << endl;
   }
      
   return counter;
   
} // end function GetValues

Here's BitStreamIn.h

Code:

#include "..\FEP\fep.h"
#include <fstream>

using namespace std;

class BinStreamIn : public ifstream 
{
   
public:
   BinStreamIn( const char *inputFileName );
   void ReadBytes(void *inputBytes, int numOfBytesToInput);
   template <class T> BinStreamIn & operator>> (T &inputValue);
   
}; // end class BinStreamIn


//////////////////////////////////////////////////////////////////////////////// 
// 
//  Function Name : ReadBytes 
// 
//////////////////////////////////////////////////////////////////////////////// 
void BinStreamIn::ReadBytes(void *inputBytes, int numOfBytesToInput) 
{

  // Local Variables
  bool somethingIsWrongWithTheVoidPointer = !inputBytes;
  bool invalidNumOfBytesProvided = (numOfBytesToInput <= 0);

  if (somethingIsWrongWithTheVoidPointer || invalidNumOfBytesProvided) 
  {
    return;
  } // end if
  else 
  {
    read((char *)inputBytes, numOfBytesToInput);
  } // end else

} // end function BinStreamIn::ReadBytes

//////////////////////////////////////////////////////////////////////////////// 
// 
//  Function Name : operator>> 
// 
//////////////////////////////////////////////////////////////////////////////// 
template <class T> BinStreamIn & BinStreamIn::operator>> (T &inputValue) 
{
   BinStreamIn::ReadBytes( &inputValue, sizeof(inputValue) );
  return *this;

}; // end function BinStreamIn::operator>>

I did find that if I use the alternate method of calling the overloaded >> operator like this:

Code:

      dataFile.operator >>(values[counter]);

that I get a different error.

Code:

DataStreamReader.cpp(28) : error C2893: Failed to specialize function template 'class BinStreamIn &__thiscall BinStreamIn::operator >>(T &)'
        With the following template arguments:
        'unsigned short'

Hopefully that's enough code to spot a problem.