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ClassA myA;

((ClassB*)(&myA))(...);

can i convert ClassA to ClassB (ClassB : public ClassA)? without using new, and RTTI (typecast)

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classA myA;
classB* myB = static_cast<classB*>(myA);

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ClassA MyClass
#ifdef ...
    ClassB* MyClassChild = (static_cast<ClassB*>(MyClass))(...); // Call Constructor
    MyClass = MyClassChild;
#endif
MyClass.virtualFunction();

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ClassA BaseClass; // ClassA only contains virtual functions
#ifdef _USE_CLASS_B
  // Want to change ClassA to ClassB (ClassB : public ClassA)
#elif   _USE_CLASS_C
  // Want to change ClassA to ClassC (ClassC : public ClassA)
#endif
BaseClass.VirtualFunction(); // Call the function (BaseClass now is ClassB or ClassC, Can't be ClassA becuase all functions are virtual)

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user@localhost:/void/cygwin/osdev/zbz$ cat ~/tst.cpp
#include <stdlib.h>
#include <stdio.h>

class animal {
public:
	virtual void foo(void) { printf("animal::foo()"); };
	void bar(void) { printf("animal::bar()"); };
};

class bear
: public animal {
public:
	void foo(void) { printf("bear::foo()"); };
	void bar(void) { printf("bear::bar()"); };
};

/* The purpose of polymorphism (as done in C++) is to allow you to use a
 * pointer or reference to a base class and assign it to an instance of a
 * derived class, and even though the _type_ of the pointer being used may be
 * of the base class, if the object being pointed to is a derived class with an
 * implementation of one of the base class's virtual functions, the _derived_
 * class object's implementation of that function will be called, in spite of
 * type of the pointer being used to refer to the object. Consider:
 **/
animal *pAnim;
bear bearArr[16];

int main(void)
{
	for (int i=0; i<16; i++)
	{
		pAnim = static_cast<animal *>( &bearArr[i] );
		pAnim->foo();
	};
	return EXIT_SUCCESS;
}

user@localhost:/void/cygwin/osdev/zbz$
user@localhost:/void/cygwin/osdev/zbz$ g++ ~/tst.cpp -o ~/tst
user@localhost:/void/cygwin/osdev/zbz$ ~/tst
bear::foo()bear::foo()bear::foo()bear::foo()bear::foo()bear::foo()bear::foo()bear::foo()bear::foo()bear::foo()bear::foo()bear::foo()bear::foo()bear::foo()bear::foo()bear::foo()user@localhost:/void/cygwin/osdev/zbz$ 

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ClassA BaseClass; // ClassA only contains virtual functions
#ifdef _USE_CLASS_B
  // Want to change ClassA to ClassB (ClassB : public ClassA)
#elif   _USE_CLASS_C
  // Want to change ClassA to ClassC (ClassC : public ClassA)
#endif
BaseClass.VirtualFunction(); // Call the function (BaseClass now is ClassB or ClassC, Can't be ClassA becuase all functions are virtual)

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#ifdef _USE_CLASS_B
ClassB BaseClass();
#else
ClassC BaseClass();
#endif
BaseClass.VirtualFunction();

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ClassA *BaseClass = 0;
ClassB b;
ClassC c;
if(use_class_b) {
    BaseClass = &b;
} else {
    BaseClass = &c;
}

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class ClassB {};

class ClassC {};

#ifdef SOMETHING
typedef ClassB ClassA;
#else
typedef ClassC ClassA;
#endif

ClassA object;

object.function();