Instantiate virtual classes into main

Hello

For someone new to not only C++ but also C, it would be best to start with one or more tutorials. That would help you understand the most important basics and some basic program that you can then extend to try out more advanced stuff by yourself.

Here's a decent tutorial on Cplusplus.com, which is also a good reference site for advanced language features: C++ language tutorial

As to your questions:
1. a class that has one or more pure virtual functions cannot be instantiated. This is what is called a (pure) virtual, or 'abstract' class. Typically you use such classes if all you want to do is describing a specific behaviour, rather than the entire definition of the real class, or if you want to describe behaviour that is shared by several related classes.

Example:

// file Shape.h

// class Shape describes some geometric object
// it may be drawn, but how to draw a shape depends on the specific type of shape
// therefore we define this class as pure virtual, and leave the drawing to the
// concrete shape classes:
class Shape {
public:
   virtual void draw() = 0; // pure virtual or 'abstract' member function
};

// file Triangle.h

#include "Shape.h" // read declaration of the base class Shape

// class triangle
class Triangle : public Shape {
public:
   virtual void draw(); // this function will draw a triangle
};

// file Triangle.cpp

#include "Triangle.h"

// implementation of the Triangle member functions
void Triangle::draw() {
   // draw some triangle here ...
}

(and now do the same for other types of shapes, e. g. Circle, Square)

// file main.cpp

#include "Shape.h"
#include "Triangle.h"
#include "Circle.h"
#include "Square.h"

int main() {
   Shape* shapes[3];
   // construct the shape objects
   shapes[0] = new Triangle();
   shapes[1] = new Square();
   shapes[2] = new Circle();

   // now draw all shapes:
   for (int i = 0; i < 3; ++i) {
      shapes[i].draw();
   }

   // destroy the constructed objects again
   for (int i = 0; i < 3; ++i) {
      delete shapes[i];
   }

   return 0;
}

Now, if you could instantiate an object of class Shape and assign it to your shapes array, what do you think the call to draw() would do? You don't know? Well, the compiler doesn't either. That's why it is forbidden.

(rest of the questions in separate solution(s))