How to use one variable for 2 datatype inside class

Question

Dear all
i have create 2 class GDI and OGL need use use in M_VIEW as per condition
in the class M_VIEW (example below)
#define M_FLAG 1
class GDI {public: int z;};
class OGL{public: double z;};
// in class M_VIEWi need to use GDI or OGL as per user condition
class M_VIEW{
public:
#if (M_FLAG == 1)
GID UseThis;
#else
OGL UseThis;
#endif
this is work but it always it take OGL. of if i change condition it take GDI only. but i need to use it runtime as per user choice.
how to switch GDI to OGL, and OGL to GDI on runtime ;
is that possible to change M_FLAG  value on run time or is there any other way to achieve it.
i have try with polymorphism also. switch is ok but all function does not work with dll. when call function on mouse move or some other event it take base class virtual function. it doesn't goes to derived class function. don't know why?
base class function like this and does not have any variable. all function are virtual.
virtual void MoveLine(POINT pt1, POINT pt2){};
virtual void DrawLine(POINT pt1, POINT pt2){};
please help.
Thanks in Advance.

Answer 1

Well, #define, and #if are compile time only constructs.  Technically they are processed before you program is compiled (that is why they are called preprocessor directives).  If you need to support both flavors at runtime you will need a different
approach.
Inheritance/polymorphism could be a good approach here, but I don't really understand what you are trying to do well enough to say for sure.  Based on guesses about what you want, here are some thoughts.
class GDI {public: int z;};
class OGL{public: double z;};
class M_VIEW_BASE {
virtual void MoveLine(POINT pt1, POINT pt2) = 0;
virtual void DrawLine(POINT pt1, POINT pt2) = 0;
class M_VIEW_GDI {
GDI UseThis;
void MoveLine(POINT pt1, POINT pt2) override {}
void DrawLine(POINT pt1, POINT pt2) override {}
class M_VIEW_OGL {
OGL UseThis;
void MoveLine(POINT pt1, POINT pt2) override {}
void DrawLine(POINT pt1, POINT pt2) override {}
std::unique_ptr<M_VIEW_BASE> drawBase;
enum DrawMode { DrawGdi, DrawOgl };
extern "C" __declspec(dllexport) void Init(DrawMode whichMode) {
if (drawMode == DragGdi) {
drawBase.reset(new M_VIEW_GDI);
} else if (drawMode == DrawOgl) {
drawBase.reset(new M_VIEW_OGL);
} else {
throw std::runtime_exception("whoops");
extern "C" __declspec(dllexport) void MoveLine(POINT pt1, POINT pt2) {
drawBase->MoveLine(pt1, pt2);
extern "C" __declspec(dllexport) void DrawLine(POINT pt1, POINT pt2) {
drawBase->DrawLine(pt1, pt2);