Re: Linkage of namespace scope reference

Mon, 12 Jun 2006 21:38:46 CST
Alf P. Steinbach wrote:

One would expect, and I did originally expect, that when initialization
is required, internal linkage is implied, but this special case breaks
that expectation -- for no good reason that I can see.

I don't think I understand why "one would expect" any relationship
between the linkage of a name and whether an explicit initializer is
required where the name is defined. Consider:

struct foo1 { };
struct foo2 { foo2() {} };

namespace ns {

   int a; // external linkage; legal
   static int b; // internal linkage; legal

   const int c; // internal linkage; illegal
   int& d; // external linkage; illegal
   const int& e; // internal linkage; illegal

   const foo1 f1; // internal linkage; illegal
   const foo2 f2; // internal linkage; legal


All of the above are definitions.

Neither a nor b requires an explicit initializer, even though a has
external linkage and b has internal linkage. c and e have internal
linkage and require initializers, but d (with external linkage) also
requires an initializer. Finally, both f1 and f2 have internal
linkage, but only f1 requires an initializer.

The extern keyword doesn't change whether a definition requires an
initializer; it only changes whether a declaration without an
initializer is also an (ill-formed) definition.

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