I have a few questions about the linkage from the following variables. By examples of 7.1.1/7 of C++03 and experimenting with compilers (Comeau, Clang and GCC), I came to the following linkage kinds:
First
static
, thenextern
static int a; // (a) extern int a; // (b) valid, 'a' still internal
It's clear to me with accordance to section 3.5: (a) implies internal linkage. And (b) also implies internal linkage, because the name "a" is declared static (by (a)).
First
extern
, thenstatic
extern int b; // (c) static int b; // (d) invalid!
First, (c) implies external linkage. But (d) implies internal linkage because the name "b" is declared static by (d). This is invalid according to 7.1.1/7, since the linkage implied is not consistent.
First
const
, thenextern
const double pi1 = 3.14; // (e) extern const double pi1; // (f) valid and 'pi1' is internal
First, (e) implies internal linkage, because it is const, and neither declared explicit extern nor previously implied external linkage. And (f) should imply extern linkage and be an error, because it explicitly declares the name extern, but the compilers keep it internal! Why so? That's my question.
First
extern
, thenconst
extern const double pi2; // (g) const double pi2 = 3.14; // (h) valid and 'pi2' is external
Now, (g) implies external linkage because we explicitly declared extern. And (h) also implies external linkage because (g) explicitly declared extern.
I have experimentally found out the linkage for 3 and 4 with the following template (the second argument is required to have external linkage)
template<typename T, T&> struct ensure { };
ensure<const double, pi1> e1; // failed
ensure<const double, pi2> e2; // succeeded
Summary: The Discussion with Charles Bailey turned out to be quite fruitful and showed there are two possible interpretations of 3.5/3
, where the important bullet point reads
A name having namespace scope (3.3.5) has internal linkage if it is the name of
- an object or reference that is explicitly declared const and neither explicitly declared extern nor previously declared to have external linkage;
If we look at point (f)
, then the two interpretations come to different conclusions, as shown below
The first interpretation notes that
pi1
is declaredconst
but is also declaredextern
. The variable has thus external linkage.The second interpretation interpretes both occurences of "declared" to refer to the same declaration. In this way, it means that it is declared
const
, but notextern const
. We note that(e)
is declaredconst
and notextern const
, thus we givepi1
internal linkage.
Now what interpretation is correct? I can't determine from that wording, but compilers seem to interpret this the second way. In particular, if we take the first interpretation, then the last quoted part of 3.5/3
would be superfluous, because there would be no valid scenario in which a name would be declared const
and previously declared with external linkage but without an explicit extern
.