c++ - Why can an ifstream object be compared to 0? -

i'm trying learning c++ after having first learned basics 20 years ago setting aside. started picking book had learned from, then. here's 1 of examples (i've figured out has obsolete syntax):

#include <fstream.h> void main() {     ifstream infile("iocopy.cpp");     if (!infile) cerr << "couldn't open iopcopy.cpp" << endl;     ... other junk } 

his explanation of ! operator on infile "checks see if object nonzero", confusing me (what mean object zero?). decided try myself:

#include <fstream> #include <iostream> void main () {     std::ifstream f ("something"); //  if (f == 0) std::cout << "couldn't open file" << std::endl;     if (!f) std::cout << "couldn't open file" << std::endl; } 

this works expected, displaying message if file doesn't exist , nothing if does. works if uncomment line (f == 0) , comment out following line.

i think figured out ! operator here; found operator! in 1 of header files defined return fail field. i'm confused why first example (f == 0) works, though, since didn't see operator== seem apply.

so what's going on? can variable of class type compared 0? there special ifstream variables of type pointers (that compared null or 0)? or there operator== redefinition missed somewhere? i'm more used languages java it's impossible constructor return null value, i'm confused.

c++03

basic_ios (a base class of ifstream) has operator void* makes implicitly convertible void*.

0 valid value void* literal, comparing values of type void*.

c++11

basic_ios (a base class of ifstream) has explicit operator bool makes convertible bool explicit cast. example should not compile.

bool implicitly convertible int per c++ language type conversion rules, if operator bool had not been explicit, comparing values of type int.