Teach Yourself Borland C++ in 14 Days - portal

6
208 Day 6
NOTE
3. Press F12 to switch back to the form. Now choose File | Save As from the menu.
The File | Save As event handler is displayed.
4. Type code so that the File | Save As event handler looks like this:
void __fastcall TMainForm::SaveAs1Click(TObject *Sender)
{
TChild* child = dynamic_cast(ActiveMDIChild);
if (!child) return;
if (SaveDialog->Execute())
{
child->Image->Picture->SaveToFile(SaveDialog->FileName);
}
}
The code for the File | Save menu item is pretty simple. The first two lines check to see
whether an MDI child window is active. If, so the File Save dialog box is displayed. If the user
clicks OK, the image is saved to disk using the TPicture class’s SaveToFile() method.
In the preceding code you see a special C++ operator called
dynamic_cast. dynamic_cast is used to cast a pointer of a base class
to a pointer of a derived class. The ActiveMDIChild property returns
a pointer to a TForm object. What we actually need in this case is a pointer
to a TChild object (our MDI child class, derived from TForm) so that
we can access the Image property of the MDI child form.
If dynamic_cast is unable to perform the cast, it returns NULL. Attempting
to use a NULL pointer will result in an access violation, but the
debugger will conveniently point out the offending line so you know
exactly where the problem lies. This is much better than the alternative
of attempting to use the old-style cast, where a bad cast could result in
some random memory location being overwritten.
Before we go on, it would be a good idea to save the project. Choose File | Save All from the
main menu. Save Unit1 (the default name C++Builder assigns to a new unit) as ViewMain and
the project as ViewPict.
Step 3: Write Code for the Window Menu
Now we’ll add code to the Window menu. This part is simple: