Why does while(std::ifstream >> s) work?

Question

I've used statements such as this quite a bit in my C++ programming:

std::string s;
std::ifstream in("my_input.txt");
if(!in) {
    std::cerr << "File not opened" << std::endl;
    exit(1);
}
while(in >> s) {
    // Do something with s
}

What I want to know is, why does this work?

I looked at the return value of operator>>, and it's an istream object, not a boolean. How does an istream object somehow get interpreted as a bool value that can be put inside of if statements and while loops?

Answer 1

The base class std::basic_ios provides an operator bool() method that returns a boolean representing the validity of the stream. For example, if a read reached the end of file without grabbing any characters, then std::ios_base::failbit will be set in the stream. Then operator bool() will be invoked, returning !fail(), at which time extraction will stop because the condition is false.

A conditional expression represents an explicit boolean conversion, so this:

while (in >> s)

is equivalent to this

while (static_cast<bool>(in >> s))

which is equivalent to this

while ((in >> s).operator bool())

which is equivalent to

while (!(in >> s).fail())