abstract class declarations in c++

Question

Suppose foo is an abstract class in a C++ program, why is it acceptable to declare variables of type foo*, but not of type foo?

Answer 1

Because if you declare a foo you must initialize/instantiate it. If you declare a *foo, you can use it to point to instances of classes that inherit from foo but are not abstract (and thus can be instantiated)

Answer 2

You can not instantiate an abstract class. And there are differences among following declarations.

// declares only a pointer, but do not instantiate.
// So this is valid
AbstractClass *foo;

// This actually instantiate the object, so not valid
AbstractClass foo;

// This is also not valid as you are trying to new
AbstractClass *foo = new AbstractClass();

// This is valid as derived concrete class is instantiated
AbstractClass *foo = new DerivedConcreteClass();