Testing a stream object in a while statement

Hi,
I am puzzled by this fact that how a stream object be tested in a while statement. A stream object by itself does not return anything. If it does carry a "value" where is that value stored?...Could someone please explain
For example

Code:

ifstream infile("TEST.TXT");
while(infile)
{
//do something
}

How does a stream object take "values" and how does it get changed when certain operations are done on this object.
Also, do all objects carry a "value"?

Thanks
sudar

A stream object by itself does "return" something. In a boolean context, it evaluates to true if all is fine, or false if an error occurred or it's at the end of the stream.

Originally Posted by sudar

Also, do all objects carry a "value"?

In the case of streams that works via a conversion operator.
something like this

Code:

#include <iostream>
using namespace std;

class xstream {
public:
    xstream() : flag(0) { }
    void set_error() { flag = 1; }
    operator const void *() { return flag ? 0 : this; } 
private:
    int flag;
};

int main() {
   xstream m;
   if ( m )
        cout << "ok." << endl;
   else
        cout << "error." << endl;

   m.set_error();  // simulate error
   
   if ( m )
        cout << "ok." << endl;
   else
        cout << "error." << endl;
}

Kurt

Technically the stream operator will return !fail(). This means that if the stream is at the end of the file, even if it sets the eofbit, it will still return true unless the failbit is also set.

It just so happens that most often when a read causes the eofbit to be set it does so during failed read which also sets failbit.

True. My fault.

Hi,
Thanks for the replies...so...is this true for non-stream objects too?...In other words, any object of any class will return a value when tested?

No, this is only true of classes that implement this technique. The stream classes are just one group of classes that do. Most classes do not.