Convert class object to bool

Hi,
I was wondering how I could achieve the syntax like file streams for my classes.

Code:

---

std::fstream fin(sth);
if (fin)//?

---

The constructor would set an error flag. This would return true if error_flag == 0.
Should I overload an operator and which (I tried !)?

The fstream has an automatic conversion to void*, and that void* evaluates to true for non-zero and false for zero. So you would overload operator void*() and have it return 0 for a failed state.

Well, thanks.

I googled for "bool operator()" and it turned out "operator bool()" was what I was looking for :) Never heard of that before.

That will work, too. There is a reason that fstreams use the operator void* instead of bool. I don't remember it exactly, though. My guess is that assigning an fstream to a bool would happen automatically, which you don't really want. Having an automatic conversion to void* is much less likely to cause errors in your program. For example, if you accidentaly send an instance of your class to a function expecting a bool, the compiler won't complain, but your code might not work.

I believe the reason is that operator bool() would allow implicit casting to int, which may not be desirable.

Ok thanks. I got this and it is working nicely.

Code:

---

Maze::operator void*()
{
    return errorState ? 0 : this;
}

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Learnt something new today, although it's just "syntactic sugar" :)

The Boost libraries have a very special technique for converting to boolean, which avoids the problems of void* too. I can't remember it, though.

This is it, I believe:
http://www.artima.com/cppsource/safebool.html