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Why put the word "Virtual" If it works without it?
What does it mean "Virtual" ?
Code:#include <wx/app.h>
#include <wx/frame.h>
class FrmMain: public wxFrame {
public:
FrmMain(): wxFrame(NULL, wxID_ANY, "2019") {
}
};
class MyProgram: public wxApp {
// virtual bool OnInit() { // Why put the word "Virtual" If it works without it?
bool OnInit() {
(new FrmMain)->Show();
return true;
}
};
wxIMPLEMENT_APP(MyProgram);
wxWidgets is C++, so this thread shall be moved to the C++ programming forum.
As for your question: you need to learn about inheritance and polymorphism in C++. This is typically covered by introductory material on C++ classes.
With full acknowledgement of @laserlight's point, the declaration of wxApp:OnInit is declared as virtual, so the function in the derived class will be virtual. While it is optional, there's more to the story.
In C++11 the keyword override is used to declare the derived OnInit to indicate the author (that's you in this case) assumes the base declared that function is virtual (and thus issue an compile time error if that is not true).
Using override is a good idea to ensure that you are getting what you expect, that in this case a call to the base OnInit will call your derived version through the virtual function mechanism. Without the specification of override, you may not recognize minor differences in the declaration which effectively disconnect the virtual mechanism.
The older style, which wxWidgets is written to support (for portability among a great many compilers, even much older ones), left some ambiguity which brought up subtle bugs.
So, it isn't so much whether you declare this function as virtual or not - if the signature is the exact match, the base makes it virtual (your use of it is merely an agreement of this fact in your code), it is whether or not you recognize that you are expecting and dependent upon the virtual call mechanism to work in order to call your OnInit, as opposed to some default in the base.
The more modern use of override is, therefore, capable of helping you ensure that the override is correctly matching the base virtual function as you should expect.
This is where @laserlight's point is key. It is something you must learn to recognize as you consume libraries which depend upon the virtual function mechanism for extension, as wxWidgets (and many others) does.
I'm sorry, that was a lack of attention! I know the difference between C and C ++ and I believed I was even posting to the wxWidgets forum itself. Thanks for correcting by moving the topic!Quote:
Originally Posted by laserlight
Thanks for the info, they are very useful!Quote:
Originally Posted by Niccolo