istream::getline return type

It returns a stream so that we can chain the operation.

But when you use an object in a boolean context the compiler looks for an conversion operator that can convert it into a type that can be used in the boolean context.

C++11

In this case stream has explicit operator bool() const. When called it checks the error flags. If either failbit or badbit are set then it returns false otherwise it returns true.

C++03

In this case stream has operator void*() const. As this results in a pointer it can be used in a boolean context. When called it checks the error flags. If either failbit or badbit are set then it returns NULL which is equivalent to FALSE otherwise it returns a pointer to self (or something else valid though you should not use this fact)).

Usage

So you can use a stream in any context that would require a boolean test:

if (stream >> x)
{    
}

while(stream)
{
    /* do Stuff */
}

Note: It is bad idea to test the stream on the outside and then read/write to it inside the body of the conditional/loop statement. This is because the act of reading may make the stream bad. It is usually better to do the read as part of the test.

while(std::getline(steam, line))
{
    // The read worked and line is valid.
}

Look from reference. The istream returned from getline is converted to bool by implicit conversion to check success of operation. That conversion makes usage of if(mystream.getline(a,b)) into shorthand for if(!mystream.getline(a,b).fail()).

It returns the stream itself. The stream can convert (through void*) to bool indicating its state. In this example, your while loop will terminate when the stream's conversion to bool goes "false", which happens when your stream enters an error state. In your code, it's most likely to occur when there was an attempt to read past the end of the file. In short, it'll read as much as there is, and then stop.