g++ 4.1.2 compiler error - c++

Question

I have the following code (stripped down version from actual project to reproduce
the issue) that results in a compiler error on RHEL5 (g++ version 4.1.2):
----------- driver (test.cpp)--------------
#include <iostream>
#include <classa.hpp>
#include <func.hpp>
namespace globals {
static int kth(const A& a) {
return kth(a.ival());
}
}
using namespace globals;
int main() {
A a;
std::cout << func(a) << std::endl;
return 0;
}
----------class A (classa.hpp)------------
class A {
public:
A():val(0){}
const int ival() const {return val;}
private:
int val;
};
------- namespace globals (func.hpp) ------
namespace globals {
int kth(const int& c) {
return c;
}
template <class T>
int func(const T& key) {
return kth(key);
}
}
--------------------------------------------
Compiling it using g++ 4.1.2 gives me the following error:
func.hpp: In function ‘int globals::func(const T&) [with T = A]’:
test.cpp:15: instantiated from here
func.hpp:8: error: invalid initialization of reference of type ‘const int&’ from
expression of type ‘const A’
func.hpp:2: error: in passing argument 1 of ‘int globals::kth(const int&)’
Same code compiles and runs perfectly fine on RHEL4 (g++ version 3.4.6)! Any explanations/ideas/suggestions on how to resolve this error(?) on RHEL5 will
be much appreciated!
Edit:
Thanks Sergey. That is the obvious solution that I am aware of already. But I forgot to add that the restriction is that func.hpp cannot be edited (for e.g., its 3rd party write-protected). Any workarounds?

Answer 1

Here's what happens. When the function func() is defined, the compiler doesn't know about the function kth(const A&) yet because it is defined later in the code. So when it encounters a reference to kth() inside func(), it assumes that it is a reference to kth(const int&). Now when func() is actually instantiated, it fails to compile it because T is A, not int. I am not sure why it works in another version of the compiler, but I think it is because it actually starts resolving references when a template function is instantiated, not when it is declared. But this looks like a bug in the older version because with such behavior a function definition changes depending on where it is instantiated from, which is very confusing.
The only way to fix your code that it works with any compiler would be to put the definition of kth(const A&) between kth(const int&) and func() or a forward declaration of kth(const A&) somewhere above func().
Update
With the restriction of not editing func.hpp the best workaround I can think of is to create a custom header file with something like this:
#include <classa.hpp>
namespace globals {
static int kth(const A& a); // defined later, but used by func.hpp
}
#include <func.hpp>
I also don't see why kth(const A&) is defined as static, but used by a global header. I'd rather put it into the classa.cpp and its declaration into the classa.hpp. But this may be some design feature or artifact I am not aware of.