strange error message

Hi everyone,

Can someone explain me the weird error message that I got from this
simple code?

struct VTKP {
	std::ostream& os_;
	VTKP(std::ostream& os) : os_(os) {}
	void operator()(int i) {
		os_<<i<<endl;
	}
};

int main(int argc, char *argv[]) {

	std::ofstream dout;
	dout.open("domain.vtp");
	dout<<"<?xml version=\"1.0\"?>\n";
	dout<<"<VTKFile type=\"PolyData\" version=\"0.1\" byte_order=
\"LittleEndian\">\n";
	dout<<"<PolyData>\n";
	dout<<"<Piece NumberOfPoints=\"";
	VTKP(dout)(10);  // <<<<<<-------------------------------- error
....

main.cxx: In function 'int main(int, char**)':
main.cxx:94: error: conflicting declaration 'VTKP dout'
main.cxx:88: error: 'dout' has a previous declaration as
'std::ofstream dout'
make: *** [main.o] Error 1

and then I change that problematic line with

(VTKP(dout)(10));

and everything goes fine. I'm using GCC v4.3.

Thank you all,

aa