Re: Linkage of namespace scope reference

Wed, 14 Jun 2006 13:28:35 CST
Alf P. Steinbach wrote:

Formally that's backwards, because for a reference it's the initializer
that determines whether the declaration is a definition, not the other
way around.

It has nothing to do with whether the name being declared is a
reference, it has to do with whether the name being declared is
explicitly declared extern.

Any declaration with the extern keyword is also a definition if and
only if an explicit initializer is present. Declarations without the
extern keyword are always definitions. (I'm assuming we're just
talking about variable declarations; static data members, functions and
types follow...sigh...different rules.)

I wonder if some confusion is caused by the assumption that keyword
extern has no effect other than to give the declared name external
linkage. This is a false assumption: it changes the meaning of the

  int i; // declares & defines i
  extern int j; // declares but doesn't define j

Both i and j have external linkage. "extern" says both (1) give this
name external linkage and (2) treat this as a definition if and only if
I provide an explicit initializer.

The 'const' for something that a reference refers to is not a 'const'
for the reference itself.

The analogous case for non-reference would be a const pointer's linkage
being determined by the constness of what the pointer points to.

I think that is a mis-perception that comes from thinking of references
as automatically dereferenced pointers, rather than as what the
standard says they are: pure aliases. Consider:

   const int a = 0;
   const int& b = a;

"a" names a const object. "b" also names a const object (as it
happens, the same one). They're alike. It's non-sensical to think of
"b" as naming "the reference", because no such object exists to name.

(Of course, in some situations, the implementation will use a pointer
"under the hood," e.g. for reference members. But that's an
implementation detail, irrelevant to the *concept* of references.)

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