news:0d90fff8-4b40-477f-95e3-7cff371f1706@a12g2000yqk.googlegroups.com...
"Michael Doubez" <michael.dou...@free.fr> wrote in message
On 25 mar, 01:04, "Paul" <pchris...@yahoo.co.uk> wrote:
"Michael Doubez" <michael.dou...@free.fr> wrote in message
On 24 mar, 19:07, "Paul" <pchris...@yahoo.co.uk> wrote:
AFAICT you do not need:
template< class T > int const Foo<T>::N; /* This is instanciated
implicitly
with Foo<Bar> f anyway*/
--No it is not implicity instanciated without a definition.
--It is true that I don't really need it, provided I don't take a
--reference to it: the compiler directly reuse the value specified at
--declaration time; but it is formally UB.
How would taking a reference make any difference?
Does the template make any differenece to if you just did:
struct Foo{
static int const N = 42;
};
Try it out:
#include <iostream>
struct Foo{
// link fails with this line
static int const N = 42;
// compiles with this line
// enum { N = 42 };
};
void print( int const & v ) { std::cout<<v<<'\n';}
int main() {
print(Foo::N);
}
I did try it out after I posted, and it worked just fine, what compiler
did
you say to avoid again? :-)
[snip]
When I go onto my newer VS2010 these compile just fine , so that old
command
line compiler I've been using to test your code isn't the greatest but it
seems to compile/link your code fine without
template< class T > int const Foo<T>::N;
It doesn't compile on codepad (And comeau doesn't link so it is of no
help here).
It doesn't either on gcc (up to 4.3.3)
The standard is quite clear on the subject with the one definition
rule and a static data member is always a declaration within the class
scope (?9.4.2/2)
"The declaration of a /static/ data member in its class is not a
definition [...].
And more precisely ?9.4.2/4:
"If a static data member is of const integral or const enumeration
type, its declaration in the class
definition can specify a constant-initializer which shall be an
integral constant expression (5.19). In that
case, the member can appear in integral constant expressions. *The
member shall still be defined in a namespace scope if it is used in
the program* and the namespace scope definition shall not contain an
initializer."
This requirement still holds in the proposal for c++0x.
It may be a compiler extension or perhaps they don't consider this as
"used in the program" (the value is used but not the variable, it
could make sense).
So amybe there is something wrong with my compiler then . :)