Работа с файлом через класс

#ifndef ___TEXT_FILE___
#define ___TEXT_FILE___
 
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
 
class CTextFile
{
    public:
        CTextFile(string fname);
        CTextFile(const CTextFile & obj);
          ~CTextFile();
 
       char buf();
       int get_letter_count();
       
    private:
       char    _buf;
       string  _file_name;
       fstream _file;
 
};
#endif

CTextFile::CTextFile(string fname)
{
    _file_name = fname;
    _file = fstream(_file_name);
}
 
/* ......... */

CTextFile::CTextFile(string fname)
    : _file(fname)
    , _file_name(fname)
{
    _file.open();
}

g++ -c main.cpp
g++ -c CTextFile.cpp
CTextFile.cpp: In constructor `CTextFile::CTextFile(std::string)':
CTextFile.cpp:5: error: no matching function for call to `std::basic_fstream<cha                                                                     r, std::char_traits<char> >::basic_fstream(std::string&)'
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/iosfwd:96: note: candidates are: s                                                                     td::basic_fstream<char, std::char_traits<char> >::basic_fstream(const std::basic                                                                     _fstream<char, std::char_traits<char> >&)

CTextFile::CTextFile(string fname)
    : _file(fname.c_str())
    , _file_name(fname)
{
    _file.open();
}

CTextFile.cpp: In constructor `CTextFile::CTextFile(std::string)':
CTextFile.cpp:6: error: no matching function for call to `std::basic_ifstream<char, std::char_traits<char> >::open()'
/usr/lib/gcc/i686-pc-cygwin/3.4.4/include/c++/fstream:570: note: candidates are: void std::basic_ifstream<_CharT, _Traits>::open(const char*, std::_Ios_Openmode) [with _CharT = char, _Traits = std::char_traits<char>]

CTextFile::CTextFile(string fname)
{
    _file_name = fname;
    _file.open(_file_name);
}

int main(int argc, char ** argv)
{
    char buf;
    ifstream my_txt;
    my_txt.open(argv[1]);
 
    int number(0);
    while(my_txt.get(buf))
        ++number;
    cout << number;
 
    my_txt.close();
    return 0;
}

CTextFile::CTextFile(string fname): _file_name(fname), _file(fname.c_str())
{
    //_file.open();
}