Implement babel on c++ but it cannot convert the chemical file

Question

I'm jira. I try to implement babel in c++ in order to convert a file of chemical structures in 'mol' format to that in 'smiles' format. I wrote the code similar to the one provided in documentation of openbabel ( http://openbabel.org/docs/dev/UseTheLibrary/CppExamples.html ) as follows.

#include <openbabel/obconversion.h>
#include <fstream>

int main(int argc, char **argv){
  std::ifstream ifs("a.mol");
  std::ofstream ofs("a.smi");

  OpenBabel::OBConversion conv(&ifs, &ofs);

  conv.setInAndOutFormats("MOL","SMI");

  int n = conv.Convert();
  std::out << n << " molecules are converted.\n"

  return 1;
}

but it always prints

0 molecules are converted.

and return an empty file "a.smi".

However, when I implement babel at the command line, e.g.

babel -imol a.mol -osmi

it works fine.

I would like ask if anyone has been experienced this kind of error and how you solved it. Because I follows all the things provided in documentation, I cannot figure out the reason of this error.

Thank you very much in advance.

Jira