13.3.3 Defining Your Own Extraction Operators

When you want input for a new type, you overload the extraction operator for it, just as you overload the insertion operator for output.

Class string defines its extraction operator in the following code example:

istream& operator>> (istream& istr, string& input)
{
    const int maxline = 256;
    char holder[maxline];
    istr.get(holder, maxline, ”\n’);
    input = holder;
    return istr;
}

The get function reads characters from the input stream istr and stores them in holder until maxline-1 characters have been read, a new line is encountered, or EOF, whichever happens first. The data in holder is then null-terminated. Finally, the characters in holder are copied into the target string.

By convention, an extractor converts characters from its first argument (in this case, istream& istr), stores them in its second argument, which is always a reference, and returns its first argument. The second argument must be a reference because an extractor is meant to store the input value in its second argument.