gp_Ax1&gp_Ax2

Hello, a very simple question. when I defined an gp_Ax1/gp_Ax2 object as following:\

gp_Ax1 myAx1;

and I have include the file, but after I compile, the compiler says: it uses undefined class "gp_Ax1", but when I defined like this: gp_Ax1 myAx1(); it's ok, but when I try to use method of this object, it cannot.

same happens when I try to define 'gp_Ax2'. could you tell me how this happens? by the way, I use VC++6.0 in windows.

thanks.