Question

I have a set of enum values defined in the “Foo” class (below).

namespace Fii
{
    class Foo 
    {
        struct Bar
        {
            enum Baz
            {
                BAZ1,
                BAZ2, 
                BAZ3
            };
        };
    };
};

I am using a struct to reduce the scope of Baz enum values as well as show there are a group of related values.

My objective is to assign a value from an enum type to a variable. Using the class definition above, one can do this:

Fii::Foo::Bar::Baz myValue = Fii::Foo::Bar::BAZ1 (Works in both C++98 and C++11)

However, I feel that:

• At first glance, myValue seems to be initialized as a Fii::Foo::Bar but this is just because enum are a hack to group related constants in the parent (Bar in this case)

To improve readiness, I I re-factored the code to:

namespace Fii
{
    class Foo 
    {
        enum Baz
        {
            BAZ1,
            BAZ2, 
            BAZ3
        };
    };
};

Using this new class definition, one can do this:

Fii::Foo::Baz myValue = Fii::Foo::Baz::BAZ1 (Works in C++11 only)
Fii::Foo::Baz myValue = Fii::Foo::BAZ1 (Should work on C++98 and C++11 - not tested)

Q1) Why is Fii::Foo::Bar::Baz myValue = Fii::Foo::Baz::BAZ1 only working on C++11 ?

Q2) In C++98, is there a way to write Fii::Foo::Baz myValue = Fii::Foo::Baz::BAZ1 ? You are allowed to make any changes you like in the class definition.

Environment:
– Clang compiler with C++11 support
– Xcode 4
– Mac OS OS 10.8

Answer 1

juanchopanza’s answer’s valid for Q1…

Q2: In C++98, is there a way to write Fii::Foo::Baz myValue = Fii::Foo::Baz::BAZ1 ? You are allowed to make any changes you like in the class definition.

Something like:

namespace Fii
{
    class Foo
    {
        class Baz
        {
          public:
            enum E { BAZ1, BAZ2, BAZ3 };
            Baz(E e) : e_(e) { }
            operator const E() const { return e_; }
          private:
            E e_;
        };
    };
}

Explanation: for Fii::Foo::Baz::BAZ1 to be a valid reference to an enumeration in C++03, Baz must be a namespace or class/struct/union. But, we’re trying to make it seem as if Baz itself is the enumeration type, with BAZ1 being one of the available values. To do that, we must make Baz a user-defined-type (a class/struct) capable of storing any of the enumerations declared within its scope. Therefore, we add a data member to record the current value, a constructor to set the value, an operator to expose the enumeration value implicitly so you’d don’t need to code explicit references to e_ everywhere in the code using Baz objects or call some get() const function.