Question

Possible Duplicate:
Calling virtual function of derived class from base class constructor?

I have the following files (forgive typos, I rewriting the code fast from memory)

BaseReader(){
    openFile();
}

void BaseReader::openFile(){
    //logic to open file
}

Open file is declared as virtual public method (it was protected, but I swtiched it to public while trying to figure out what is wrong) in the .h. SortReader is defined as:

class SortReader: public BaseReader{
    public:
            SortReader();
            void openFile();
    };

with the following code:

SortReader::SortReader(): BaseReader(){}

SortReader::openFile(){
    sortFile();
    BaseReader::openFile();
}

When I Try constructing a SortReader object the sortFile method is never called. I can walk through it in a debugger and watch the SortReader call the BaseReader constructor The BaseReader constructor calls openFile which calls the BaseReader version of openFile. I want it to call SortReader’s implimentation of open file. What do I need to do to have that happen?

Answer 1

You can’t. Until the constructor finishes, the object is not yet fully constructed. In general, calling virtual methods from a constructor is a bad idea.

You can delegate the logic to a separate non-virtual method:

SortReader::openFile(){
    sortFileInternal();     //not virtual
                            //defined in SortReader
    BaseReader::openFile();
}

SortReader::sortFile()      //the virtual method
{
    sortFileInternal();
}