Re: explicit enum

"Andrei Polushin" <>
Thu, 1 Jun 2006 00:26:52 CST
Tom1s wrote:

Anyone know if they're considering adding "explicit enum" to the language?
I've read about such a proposal a few times. The essence of it is that when
you write:

explicit enum Colour { red, green, blue };

"red", "green" and "blue" aren't in the same scope as "Colour"; you've to
qualify them:

int main()
    int k = red; /* Error */
    int k = Colour::red;

It might be better to adapt recently added Java 5 enums for C++, see

For example, you Colour might look like this:

  struct enum Color { red, green, blue };

  void f()
    int k = Color::red;

Note that I would propose "struct enum" or "class enum" instead of
"explicit enum" syntax.

In addition, it should be possible to support non-trivial constructors:

  struct enum Color {
    red = 0xFF0000, // constructed by Color(int)
    green(0x00FF00), // constructed by Color(int)
    blue(0, 0, 0xFF); // constructed by Color(int, int, int)

    const int value;

    Color(int value) : value(value) {}
    Color(int r, int g, int b) : value((r << 16) | (g << 8) | b)) {}

    operator int() { return value; }

Moreover, there should be constant-specific methods (implemented as by
derived classes):

  struct enum Color {
    red = 0xFF0000 { const char* name() { return "red" } },
    green = 0x00FF00 { const char* name() { return "green" } },
    blue = 0x0000FF { const char* name() { return "blue" } };

  void f()
    cout << << endl; // prints "red"

In particular, the constant-specific name() method should be declared
implicitly, it is the most desired feature.

The implementation of this syntax is trivial.

Andrei Polushin

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