Hello,
Consider the following piece of C++ code:
- --------------------------------
#include <boost/functional/hash.hpp>
#include <cstddef>
struct A {
int n;
};
size_t hash_value(A v)
{
return boost::hash_value(v.n);
}
size_t hash_value(long long int v)
{
size_t seed = 0;
boost::hash_combine(seed, static_cast<long int>(v & 0xffff));
boost::hash_combine(seed, static_cast<long int>(v >> 32));
return seed;
}
template<class T>
size_t calc_hash(T v)
{
size_t seed = 0;
boost::hash_combine(seed, v);
return seed;
}
template size_t calc_hash(A);
template size_t calc_hash(long long int);
- --------------------------------
Now, calc_hash() for some type simply calls hash_combine() for it, which will
internally resolve to calling hash_value() for the type (I can provide a
minimal version with no dependence on the Boost include files if someone thinks
that's a problem). So the compiler needs to find the proper definition of
hash_value() for that type.
Earlier versions of g++ (say, 3.2 which I have handy here) didn't quite
conform to the standard, so the way to make the above code work was to declare
the above hash_value() functions inside the boost namespace. But these days g++
has ADL, which means that hash_value() instead has to be declared in the same
namespace as the type. In the code I'm posting above, this works fine for A;
even if I move it inside some namespace, as long as I also move hash_value(A)
inside the same namespace, the code will compile.
But what about long long int? When I try to compile the above with g++ 4.1, I
get:
error: call of overloaded 'hash_value(const long long int&)' is ambiguous
followed by a list of candidate functions, which includes all the ones declared
in the header file, but not the one declared in this file. Again, in g++ 3.2
this compiles as long as hash_value(long long) is declared inside the boost
namespace too. But with 4.1 I can't seem to be able to make it compile at all.
So is this a bug, or am I doing something wrong here?
Thanks,
Vasilis
--
Summary: argument dependent lookup: what about built-in types?
Product: gcc
Version: 4.1.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: v dot vasaitis at sms dot ed dot ac dot uk
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29618
