Calling enums in another class in C++

Question

If we define two C++ classes. One is:

abstract class A {
  public:
    enum E {F, G, H;}
};

Another is class B, and how I can use the enum E in class A then? Assuming both B and A are in the same namespace. I know in C# we can use something like:

A.E

directly, but seems that is not the case of C++.

Solution 1

In C++, . and -> are for accessing a member of this particular instance of an A object. :: is for accessing things in the scope of class A. This includes statics, enums, and function pointers.

So in C++ you want A::F or A::G if you want enum value.

for the enum type you do A::E

class A
{
public:
    enum E { F , G , H};
    virtual ~A() = 0; //a pure virtual function to make the class abstract
};

int main()
{
    A::E x; // x is declared as the enum
    x = A::F; // x is assigned a particular enum value

    return 0;
}

Also to make the class abstract you provide a pure virtual function.

Solution 2

C# has single . operator. C++ has multiple: ., -> and ::. For classes you use the last one.

Therefore, it is going to be A::E.

Remember also, that in C++ the enum's constant go to parent namespace, not the one of enum like it is in C#. Therefore, in C# you would have A.E.F, but in C++ you have A::F.

Comments

[Mooing Duck]

Added a concept that I thought was meaningful and missing from your otherwise good answer.

[parapura rajkumar]

@MD.. Thanks... I hope from my anwser it is clear that you don't do A::E::F which is common mistake

[Mooing Duck]

As of C++11, that is now allowed. (And I've always done that anyway, since MSVC allowed it as an extension)

[Raghav55]

I was just curious to know why destructor was made pure virtual function?

[Krizz]

It is more of a question to the answer above. But I guess this is a way to prevent class from being instantiated. Never used that myself though.