Why can’t abstract classes be instantiated in C++?

Abstract class is a special type of class that is designed to be used as a base class or interface for other classes, and cannot be directly instantiated. This is mainly due to several reasons:

1. Abstract classes contain pure virtual functions: an abstract class defines at least one pure virtual function (a virtual function without a specific implementation), which means it cannot be instantiated. Pure virtual functions need to be implemented in derived classes, so abstract classes can only exist as base classes for derived classes.
2. Abstract classes may lack concrete implementations: Abstract classes may contain some pure virtual functions, which do not have a specific implementation. If an abstract class can be instantiated, these functions cannot be called because they are not implemented. To avoid this situation, C++ specifies that abstract classes cannot be directly instantiated.
3. Abstract classes have the significance of instantiation: they are a conceptual abstraction that represents a generic behavior or characteristic. They do not have specific instances, but exist as base classes or interfaces for other classes to implement concrete functionality. Therefore, instantiating an abstract class does not align with its design intent.

In conclusion, abstract classes cannot be instantiated to ensure they maintain consistent abstract characteristics and can have specific functionality implemented through derived classes. This helps improve the code’s scalability and maintainability, making it clearer and more understandable.