Re: extending c++ classes and enumerations

"James Kanze" <>
8 Dec 2006 17:03:00 -0500
<> wrote:

James Kanze skrev:

In the case of enum's, of course, I doubt that it's
implementable. The compiler needs to know all of the values of
an enum in order to know the size of an instance of the enum (or
even the size of a pointer to an instance, on some machines).

In practice, this is barely a problem.

In practice, it may or may not be a problem, and the compiler
has no way of knowing in advance. The result is that it simply
isn't implementable. (You're not the first person to request

The smallest size an enum can
have is char, but it is plenty of room for many values.

But not necessarily. How is the compiler supposed to handle
something like:


    enum X { a = 0, b = 0x01, c = 0x02 } ;


    enum X { d = 0x800000000000UL } ;

If it only sees module1.hh, it is not allowed to make X larger
than an int; on my machine, it could be 1, 2 or 4 bytes. If it
sees module2.hh, it is required to make X the size of a long, 8
bytes. If some code in module1 defines an X, and passes a
pointer to it to code in module2, and the code in module 2
writes through the pointer, there are going to be serious

And one could question, why not e.g.

int enum ThreadState {

is allowed.

That's another possibility. That the user be allowed to specify
the size of the enum.

James Kanze (GABI Software)
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