How to check if stringstream>>string will put nothing on the string?

Question

For example, when parsing a text file, some times this file have stuff like this:

keyword a string here
keyword another string
keyword 
keyword again a string

Note that the 3th line have an empty string (nothing or white spaces).. The thing is that when you do stringstream>>laststring, and stringstream have an empty string (null or just white space), it will not overwrite the "laststring", it will do nothing. Theres anyway to check this situation before hand? I dont want to create a temp empty string just to check it is still empty after stringstream>>, seems lame.

Solution 1

When you cannot read from stream - its state changes, so when casting to bool, it returns false:

bool read = static_cast<bool>(ss >> laststring);

Or - in if-expr:

if (ss >> laststring) 
    cout << "Just read: " << laststring;

See example

Solution 2

You can only know after trying to read whether there was something or not. What you might be able to do is to skip whitespace and see if there is a non-space in the next location:

if ((in >> std::ws).peek() != std::char_traits<char>::eof()) {
    ...
}

Given that empty strings are cheap to create, I wouldn't bother and try read the string. Note, however, that reading from streams isn't line based, i.e., in your case above you need to split the lines first or use something like std::getline() to read the second part of line.