undefined reference error while calling a function from user defined header file and it's implementation is in .cpp file

Question

What I have made is an fstreamExtension class that publically inherits fstream class .

fstreamExtension.h :

#include <fstream>
#include <string>
#include <vector>
#ifndef FSTREAMEXTENSION_H
#define FSTREAMEXTENSION_H

class fstreamExtension : public std::fstream
{
 private:

    std::string fileIdentifier;

public:

    using std::fstream::fstream;
    using std::fstream::open;

    ~fstreamExtension();

    inline void fileName (std::string&);

    inline bool exists ();

    inline unsigned long long fileSize();
};
#endif

fstreamExtension.cpp :

#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include "fstreamExtension.h"

inline void fstreamExtension::fileName (std::string& __fileIdentifier)
{
   fileIdentifier = __fileIdentifier;
}

inline bool fstreamExtension::exists ()
{
  if (FILE *file = fopen(fileIdentifier.c_str(), "r"))
  {
    fclose(file);
    return true;
  }

  else
    return false;
}


inline unsigned long long int fstreamExtension::fileSize()
{
  if(exists())
  {
    std::ifstream tempStream(fileIdentifier.c_str(), std::ios::ate |  std::ios::binary);
    unsigned long long int __size = tempStream.tellg();
    tempStream.close();
    return __size;
}

else return 0;
}

fstreamExtension::~fstreamExtension()
{
  std::fstream::close();
  std::cout << "stream closed";
}

When this code is implemented in main file as :

#include <iostream>
#include <fstream>
#include "fstreamExtension.h" 

int main()
{
  string s = "QBFdata.txt";
  fstreamExtension fs(s.c_str(), ios::in | ios::binary);
  fs.fileName(s); //error
  cout << fs.fileSize(); //error
}

There is a linker error when I call functions filename() and fileSize() .

The codeblocks species the following error :

undefined reference to fstreamExtension::fileName(std::string&)

Thanks for your help, please suggest if any structure changes are required.

Answer 1

Remove inline from function declarations and definitions to fix the linker errors.

inline makes sense for functions defined in header files. For functions defined elsewhere inline makes them unavailable to other translation units, causing the linker error which you observe.