Question

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

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.