Why would a virtual function be private?

Question

I just spotted this in some code:

class Foo { [...] private:     virtual void Bar() = 0; [...] } 

Does this have any purpose?

(I am trying to port some code from VS to G++, and this caught my attention)

Answer 1

See this Herb Sutter article as to why you'd want to do such a thing.

Answer 2

This is a pure virtual function that happens to be private. This makes it so that a derived class must implement the method. In this case Bar.

I think you may be confused b/c this is done to create "interfaces" in C++ and a lot of times people think of these as public. There are cases where you may want to define an interface that is private where a public method uses those private methods in order to ensure the order of how they are called. (I believe this is called the Template Method)

For a relatively bad example :)

 class RecordFile {     public:        RecordFile(const std::string &filename);         void process(const Record &rec)        {            // Call the derived class function to filter out            // records the derived instance of this class does            // not care about            if (filterRecord(rec))                {                writeRecordToFile(rec);                       }        };      private:        // Returns true if the record is of importance        // and should be kept        virtual bool filterRecord(const Record &rec) = 0;         void writeRecordToFile(const Record &rec); };