How is std::getline() equated with bool?

Question

I'm a std::getline(...) virgin and having consulted documentation and example at cppreference.com, I'm confused by example code such as this:

#include <sstream>
#include <string>

int main(int argc, char* argv[])
{
    std::string line;
    std::ifstream infile("sample.txt");

    while (std::getline(infile, line))
    {
        // Do stuff
    }

    return 0;
}

...specifically the while statement: while (std::getline(infile, line)).

The noted documentation says the return value of std::getline(std::basic_istream<CharT,Traits>& input, ...) is input, i.e. a reference to the first argument.

How, then, can the return value of getline be used as the while loop's condition, which needs to be of type bool?

Does std::ifstream implement an operator bool()?

‘*Does `std::ifstream` implement an `operator bool()`?*’ Why [yes it does](http://en.cppreference.com/w/cpp/io/basic_ios/operator_bool), and you can [look it up at that same website](http://en.cppreference.com/w/cpp/io/basic_ifstream). — Biffen, Oct 28 '16 at 06:05
"Does `std::ifstream` implement an `operator bool()`?" [Close. Inherited one](http://en.cppreference.com/w/cpp/io/basic_ios/operator_bool) — user4581301, Oct 28 '16 at 06:05
Good point @PaulRooney , but seriously. Who remembers that far back? — user4581301, Oct 28 '16 at 06:19

score 3 · Accepted Answer · answered Oct 28 '16 at 06:05

Does std::ifstream implement an operator bool()?

It does:

Checks whether the stream has no errors. <...> Returns true if the stream has no errors and is ready for I/O operations. Specifically, returns !fail().

This operator makes it possible to use streams and functions that return references to streams as loop conditions, resulting in the idiomatic C++ input loops such as while(stream >> value) {...} or while(getline(stream, string)){...}

How is std::getline() equated with bool?

1 Answers1