Why can't one instantiate an object of an abstract class

Here is a good language independant descritpion found on Wikipedia: http://en.wikipedia.org/wiki/Abstract_type[^].

Best Regards,

—MRB

An abstract class in C++ does not have enough information to instantiate a valid object.
Read this link for more detail: http://msdn.microsoft.com/en-us/library/c8whxhf1.aspx[^]

Hi
We can't instatiate an oject of absract class because abstract class give as informations about the behavior but there are no implementation.
Absract class should be inherited, the sub class will inherit the behavior but the implementation is a matter of the sub class.

hope that this will help.

Here's the definition of an abstract class:

#include <iostream>
// Interface to classes that can be used with iostream classes
class IStreamable {
public:
   virtual void read(std::istream& is) = 0; // abstract method!
   virtual void write(std::ostream& os) const = 0; // abstract method!
};
std::istream& operator>>(std::istream& is, IStreamable& s);
   s.read(is);
   return is;
}
std::ostream& operator<<(std::ostream& os, const IStreamable& s);
   s.write(os);
   return os;
}

This class defines, not the structure of a class, but a particular behaviour: any class that is derived from it and implements the abstract methods can interact with streams just like any of the built-in types.

For instance consider this class:

#include"IStreamable.h" // make sure you've set your include path!
class MyData : public IStreamable {
private:
   int myint;
   double mydouble;
   char mychar;
public:
   MyData(int i, double d, char c) : myint(i), mydouble(d), mychar(c) {}
   virtual void read(std::istream& is);
   virtual void write(std::ostream& os) const;
};

void MyData::read(std::istream& is) {
   is >> myint >> mydouble >> mychar;
}
void MyData::write(std::ostream& os) const {
   os << myint << ' ' << mydouble << ' ' << mychar;
   // note: don't forget to add some whitespace separators!
}

Now, if you instantiate an object of class MyData, you can easily stream it any way you like, but there is no point instantiating IStreamable, as it has neither data elements to stream, nor the function implementation required to make it work:

#include "MyData.h"

int main() {
   MyData a(3, 3.14, 'p'); // ok, a instantiates MyData
   std::cout << a << std::endl;  // ok, prints out "3 3.14 p"
//   IStreamable s; // error: cannot instantiate abstract class!
//   cout << s; // error: no implementation for IStreamable::write(std::ostream&)
   return 0;
}

As you can see it doesn't make sense to create an instance of IStreamable, and, to add insult to injury, the compiler cannot possibly do that either, as it cannot find any member function definitions.