default parameter value for a reference from a constructor

[<gongweigang@xxxxxxxxx>]

Hi, all,

I have following code:

class A { };

class B
{
 public:
    void foo( A& ra = A() ) { }
};

int main() {
   return 0;
}

If I compile with g++, it has following error message: invalid type 'A' for 
default argument to 'A&'.

But when I compile it using SunStudio compiler, it works.

I suspect the error is due to set the default value for reference "ra" with 
a constructor.

Can anyone explain whether the above code is valid in C++ standard ? If not, 
how to make it work by g++ ?

Thanks.

Michael