🚀 go-pugleaf

In article 62g@sunsystem5.informatik.tu-muenchen.de, schuenem@informatik.tu-muenchen.de (Ulf Schuenemann) writes:

>|> >class X
>|> >{
>|> >static const int fifty;
>|> >};
>|> >int X::fifty = 50;
>    ^^^^^^^^^^^^^^^^^  (*)

>|> But why not use the method that preserves the original intent of having
>|> fifty be a manifest constant?
>|> 
>|> class X {
>|> 	static const int fifty = 50;
>|> };
>|> and then in one place
>|> 	int X::fifty;

>Coz I've to designate once and forever one file to contain this 
>definition and to link it whenever I use class X.

>const int X__fifty = 50;

>It this works, why cant (*) work the same way ? The only 
>difference is, that X::fifty is a member and X__fifty isn't.

That isn't the only difference. X::fifty is (a single) global variable.
X__fifty is not global. It has static linkage, and there might be one in
every compilation unit.

The appropriate comparison would be with
	extern const int X__fifty = 50;
which would appear in exactly one place, and
	extern const int X__fifty;
every place else.

That said, the One-Definition Rule for inline member functions requires
that the compiler have some mechanism for ensuring exactly once copy of
things which have multiple definition points.
Example:

	header.h:
	========
	class X {
		virtual int f() { .... }
		// there can be only one X::f() in the whole program
	};

Templates present a similar problem, since the template instantiation
code for an external function might appear in several compilation units.

Since the compiler must have such a mechanism anyway, it seems to me
that we could eliminate the requirement for a single point of
definition of static members. I don't know of any such proposal
before the C++ committee, however.
---
Steve Clamage, stephen.clamage@eng.sun.com



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