Re: name lookup and unnamed namespaces
seanf schrieb:
Hi everyone
This code fails to compile with gcc 3.4.6 & msvc++7:
(can't find operator<<)
--------8<------------
#include <ostream>
#include <iostream>
#include <iterator>
using namespace std;
class C {};
namespace
{
ostream& operator<<(ostream& o, const C&)
{
return o;
}
}
int main()
{
ostream_iterator<C> i(cout, "\n");
i = C();
}
--------8<------------
However, it works if I put the operator<< in the global namespace.
What's going on here?
Welcome in the world of ADL ;-)
The compiler is correct here. An unnamed namespace
behaves as written as:
namespace "some_unique_identifier" {}
using namespace "some_unique_identifier";
namespace "some_unique_identifier" {
// contents
}
(see 7.3.1.1)
In short ADL (argument dependent lookup) looks in the
associated namespaces of the arguments of the function call.
The function call occurs in namespace std inside the
function member of std::ostream_iterator. The arguments
are ::C and ::std::basic_ostream<char, ::std::char_traits<char>>.
As mentioned above, C belongs to the global namespace, but
you have defined ostream& operator<<(ostream& o, const C&)
inside the unnameable namespace "some_unique_identifier",
which is such does not belong to the associated namespaces
of C, nor to those of std::basic_ostream<..>.
Although not recommended in general, you could also ensure a
successfull compilation, if you move C into the same unnamed
namespace:
namespace
{
class C {};
ostream& operator<<(ostream& o, const C&)
{
return o;
}
}
While trying to distil a minimal example, I found that this compiles:
--------8<------------
namespace N
{
template <class T> struct K
{
void f(const T& t) { g(t); }
};
}
using namespace N;
class C {};
namespace
{
void g(const C&) {}
}
int main()
{
K<C> k;
k.f(C());
}
--------8<------------
I can't seem to bridge the gap between the two examples. What am I
overlooking?
I'm quite sure that your example should also *not* compile according
to the same rules as described above. And it does not with e.g. Comeau
online.
Greetings from Bremen,
Daniel Kr?gler
--
[ See http://www.gotw.ca/resources/clcm.htm for info about ]
[ comp.lang.c++.moderated. First time posters: Do this! ]