Signaling an error in file streams in C++

Question

Hello!

I have got the following sample:

#include <iostream>
#include <fstream>
using namespace std;
int main()
{
    ifstream file;
    cout << file << endl;           // 0xbffff3e4
    file.open("no such file");
    cout << file << endl;           // 0
    cout << (file == NULL) << endl; // 1
    cout << file.fail() << endl;    // 1
}

If the file is NULL, how is it possible to call the fail member function? I am not very familiar with C++, is this normal behaviour? What am I getting wrong here?

Answer 1

Try file.good() or cast the file to bool:

file.open("no such file");
if (file)
  cout << "Open OK" << endl;
else
  cout << "Open FAILED" << endl;

Answer 2

file is an object - it cannot be null. However, ifstream has an operator void*() overload which returns 0 when the file is in a bad state. When you say (for example):

cout << file << endl;

the compiler converts this to:

cout << file.operator void*() << endl;

This conversion will be used in all sorts of places - basically anywhere that a pointer or integer type could be used. It is used when you say:

(file == NULL)

You compare the zero returned by operator void*() zero with NULL and get 1.