C++ Parent class calling a child virtual function

Question

I want a pure virtual parent class to call a child implementation of a function like so:

class parent
{
  public:
    void Read() { //read stuff }
    virtual void Process() = 0;
    parent() 
    {
        Read();
        Process();
    }
}
class child : public parent
{
  public:
    virtual void Process() { //process stuff }
    child() : parent() { }
}

int main()
{
   child c;
}

This should work, but I get an unlinked error :/ This is using VC++ 2k3

Or shouldn't it work, am I wrong?

Answer 1

The superficial problem is that you call a virtual function that's not known yet (Objects are constructed from Parent to Child, thus so are the vtables). Your compiler warned you about that.

The essential problem, as far as I can see, is that you try to reuse functionality by inheritance. This is almost always a bad idea. A design issue, so to speak :)

Essentially, you try instantiating a Template Method pattern, to separate the what from the when: first read some data (in some way), then process it (in some way).

This will probably much better work with aggregation: give the Processing function to the Template method to be called at the right time. Maybe you can even do the same for the Read functionality.

The aggregation can be done in two ways:

1. Using virtual functions (i.e. Runtime Binding)
2. Using templates (i.e. Compile Time Binding)

Example 1: runtime binding

class Data {};
class IReader    { public: virtual Data read()            = 0; };
class IProcessor { public: virtual void process( Data& d) = 0; };

class ReadNProcess {
public:
    ReadNProcess( IReader& reader, IProcessor processor ){
       processor.process( reader.read() );
    }
};

Example 2: compiletime binding

template< typename Reader, typename Writer > // definitely could use concepts here :)
class ReadNProcess {
public:
     ReadNProcess( Reader& r, Processor& p ) {
         p.process( r.read() );
     }
};

Answer 2

Title of the following article says it all: Never Call Virtual Functions during Construction or Destruction.