istream (ostream) vs. bool

Question

Here is a C++ code which reads as many words from a given text file as possible until it meets EOF.

string text;
fstream inputStream;


inputStream.open("filename.txt");

while (inputStream >> text)
    cout << text << endl;

inputStream.close();

My question is:

• what procedure exactly is performed behind on converting the condition of the while loop (i.e., inputStream >> text) into a boolean values (i.e., true or false)?

My own answer for the question is:

• To my understanding, inputStream >> text is supposed to return another (file) input stream. The stream seems to be NULL when EOF arrives. The NULL may be defined as 0, which is equivalent to false.

Does my answer make sense? Even if my answer does make sense, such conversion of InputStream to bool doesn't make me so comfortable. :)

Answer 1

what procedure exactly is performed behind on converting the condition of the while loop (i.e., inputStream >> text) into a boolean values (i.e., true or false)?

operator>> returns a reference to the stream.

In C++11 the reference is then converted to a bool by the stream's operator bool() function, which returns the equivalent of !fail().

In C++98 the same is achieved by using operator void*(), and the returned pointer is either NULL to indicate failure or a non-null pointer if fail() is false, which is then implicitly converted to a bool in the while evaluation.