Which of the following C++ statements correctly declares an abstract class ?
class A { virtual void show()=0; };
In C++ a class becomes abstract when it has at least one pure virtual function. A pure virtual function is declared with the keyword virtual and the pure specifier = 0; such a class cannot be instantiated and serves as an interface for derived classes.
The declaration class A { virtual void show()=0; }; has exactly this form: show is virtual and marked pure with =0, so A is abstract.
Writing void show()=0; without virtual is ill-formed, because only virtual functions may carry the pure specifier. Giving show() an empty body makes it an ordinary member function, so the class is concrete and can be instantiated. Omitting the return type in show()=0; is not valid C++ at all.
class A { virtual void show()=0; }; option is correct.
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