I have a design problem I am trying to overcome. Lets assume that the class structure and hierarchy is correct, and I can not change that.
I have a class called BaseWidget. It has internal nested classes Grid, and Square. Grid maintains a vector of squares. They are there to better organise drawing behaviour. BaseWidget constructs the vector of Grid and Squares in its Layout() method.
Now I have Sub-classed BaseWidget with class DerivedWidget. Derived widget has additional drawing requirements for the class BaseWidget::Square. So it would be nice and easy to sub-class BaseWidget::Square with Derived::WidgetSquare.
The problem comes from the fact that the Grids and Squares are constructed in BaseWidget::OnLayout. So when the Grid vector of squares is created I would like to say "use DerivedSquare instead of Square in the vector you are populating".
I believe my solution is to use templates but I am not sure in what capacity.
class BaseWidget : public Widget
{
protected:
void Layout(void)
void Draw(canvas c);
class Square {
Square(Rect r) : region(r) {}
Rect region;
virtual void Draw(canvas c);
};
class Grid {
std::std::vector<shared_ptr<<Square>> squares;
Rect region;
void Draw(canvas c);
};
std::vector<Grid> m_grids;
};
void Parent::Layout {
m_grids.resize(num_grids);
for (int i=0; i<num_grids; ++i) {
m_grids[i].region = some_rect;
for (int j=0; j<num_squares; ++j) {
m_grids[i].m_squares.push_back(std::make_shared<Square>(some_other_rect));
}
}
}
void BaseWidget::Draw(Canvas c)
{
for (int i = 0; i < m_grids.size(); i++) {
m_grids.Draw(c);
}
}
void Grid::Draw(Canvas c)
{
// draw some stuff here
for (int i = 0; i < m_squares.size(); i++) {
m_squares[i].Draw(c);
}
}
void Square::Draw(Canvas c)
{
// draw some stuff here
}
// new class that extends parent
class DerivedWidget : public BaseWidget
{
protected:
/*
* This class wants to do some special drawing in addition to its parents drawing
*/
class DerivedSquare : public Square {
virtual void Draw(canvas c);
};
}
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