Re: a pointer to a derived class problem
stefven blonqhern wrote:
hello all, having a problem with derived classes. i'll show by
pseudo code example. first the base class and derived classes:
class Shape { // base class for all shapes
public:
virtual void Draw() {// draw code here}
};
class Square : public Shape {// etc};
class Circle : public Shape {// etc};
class Triangle : public Shape {// etc};
then I create another class called MyClass which has a member function
ChangeShape which creates a new Shape object and assigns a pointer to
it. the member function Render is being called constantly in the
background.
class MyClass {
public:
void ChangeShape(int shape) {
// if shape = 0 then sh = Square, 1 = Circle, 2 = Triangle
// but for the sake of brevity i'll just use a square
sh = Square();
That's a VERY BAD IDEA(tm). What happens is called "slicing". The
object 'sh' is of the type 'Shape' and it is assigned only the base
class subobject from the temporary 'Square'. All the characteristics
(polymorphic included) are lost.
shPtr = □
That's not even C++. Did you mean
shPtr = &sh;
?
}
void Render() {shPtr->Draw};
Shape sh;
Shape* shPtr;
};
obviously whenever i access the member function Draw through the
Shape* (shPtr) it always calls the member for Shape not Square because
sh is of type Shape. all this might sound ridiculous?
Is that a question.
can i assign
derived classes to sh and keep the derived class type instead of them
being converted to the base class type?
No. Read up on "slicing".
my implementation is somewhat
limited by the plug-in architecture i must work within.
What does that mean?
V
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