Question

    class Message
{
    public:
        std::string getHeader (const std::string& header_name) const;
        // other methods...
};

class MessageSorter
{
    public:
        // take the field to sort by in the constructor
        MessageSorter (const std::string& field) : _field( field ) {}
        bool operator (const Message& lhs, const Message& rhs)
        {
            // get the field to sort by and make the comparison
            return lhs.getHeader( _field ) < rhs.getHeader( _field );
        }
    private:
        std::string _field;
};

std::vector<Messages> messages;
// read in messages
MessageSorter comparator;
sort( messages.begin(), messages.end(), comparator );

For this line:
bool operator (const Message& lhs, const Message& rhs)

is this right?
Should it be
bool operator() (const Message& lhs, const Message& rhs)

This code is a tutorial exmample code for Functor.
can be seen here:
http://www.cprogramming.com/tutorial/functors-function-objects-in-c++.html

Thank you

Answer 1

You got it – It’s probably a typo, it should read

bool operator()(const Message& lhs, const Message& rhs)