fstream/ios (hopefully not OT)

fstream/ios (hopefully not OT)

[Wagner Matthias]

Hi,
hopefully I am not offtopic here. If so, please ignore my req. for help
and accept my honest apologies.

A friend of mine wrote a program which used fstream by
#include<fstream>, and it didn't work. I wrote a small test-class
(dirty..)

---- CLASS LISTING
#include<cstdio>
#include<cstdlib>

//#include <ios>
#include <fstream>

using namespace::std;
class Test
{
    private:
    int m, n;

    public:
    Test() { m = n = 0; }
    Test(int a, int b=0)
    :m(a), n(b) {};
    
    friend ofstream& operator << (ofstream &ofstr, const Test &a); 
};

ofstream & operator << (ofstream &ofstr, const Test &a)
{
    ofstr << a.m;
    return ofstr;

}


---- G++
# g++ test.cpp 
test.cpp: In function `std::ofstream& operator<<(std::ofstream&, const
Test&)':
test.cpp:25: choosing `std::basic_ostream<_CharT, _Traits>& 
   std::basic_ostream<_CharT, _Traits>::operator<<(int) [with _CharT =
char, 
   _Traits = std::char_traits<char>]' over `std::ofstream& 
   operator<<(std::ofstream&, const Test&)'
test.cpp:25:   because worst conversion for the former is better than
worst 
   conversion for the latter

Now, if I include ios instead of fstream, I can compile the class
(actually a can't, as int main(void) is missing, but there is no compiler-
related error anymore).

I always thought fstream included ios, as it is a inheritive class, but as
I read through both files, I found some differences:
/usr/include/c++/3.2/ios includes iosfwd, which then includes files from
./bits/ and defines typedefs
  typedef basic_ofstream<char> 		ofstream;
ios, funnily, never includes something related to ofstream, as far as I
can see, as the structure of all those includes is _very_ chaotic, so I
assume that ios acts as some "virtual base definition" and won't work
stand alone!? "fstream" includes "istream" and "ostream" directly, so I
guess fstream is the implementation for all those typedefs in "iosfwd".
Now why is the class above not working if I include "fstream", but this
class actually compiles without errors or warnings (except for a linker
error (main again))



#include<cstdio>
#include<cstdlib>

#include <ios>
#include <fstream>

using namespace::std;
class Test
{
    private:
    int m, n;

    public:
    Test() { m = n = 0; }
    
    friend ofstream& operator << (ofstream &ofstr, const Test &a); 
};

ofstream & operator << (ofstream &ofstr, const Test &a)
{
    ofstr << a.m;
    return ofstr;

}

I only stripped one constructor of "Test"! 


May someone simply explain or send me some link how the standard
c++-headers are working? I'm quite confused...

mfg
Nec
-- 
lacrimae mortis sanguis meus sunt

Re: fstream/ios (hopefully not OT)

[John Love-Jensen]

Hi Wagner,

You have something really unusual (in an odd / bad way) in your class.  You
shouldn't use the operator << on the ofstream, you should use it on the
ostream.

If you want a binary output (not indicated by your example, but just in
case), you shouldn't use the operator << mechanism, rather you should create
a read method for input, and a write method for output.

My modified version is below, and the problem goes away.

Sincerely,
--Eljay

---- CLASS LISTING
#include <iostream>
    using std::ostream;

class Test
{
private:
    int m;
    int n;

public:
    Test()
    : m(0), n(0) { }

    Test(int a, int b = 0)
    : m(a), n(b) { }
    
    friend ostream& operator << (ostream& ofstr, const Test& a);
};

ostream& operator << (ostream& ofstr, const Test &a)
{
    ofstr << a.m;
    return ofstr;
}