虚函数绑定

Question

Hello guys have a question about virtual function binding do not know if there is such a thing so i want to ask about this .

So i have a Game State machine. GameState class is a main class for all state entities , GameStateFirstPhase is a class for entity only for those other 3 classes (StartUp,SelectServer,ConnectServer).

So what do i want to do is to split a draw function and draw all entity of those 3 classes inside a GameStateFirstPhase class, and those 3 classes have their own draw function for only specific things that are for that class, like GameStateSelectServer class would have to draw a UI Select server panel + entity of all classes which is GameStateFirstPhase class.

This is how I want to bind CurrentState draw function with a GameStateFirstPhase Draw function: My game object have a pointer to a CurrentState and call it CurrentState Draw function .

I have tried to call a Entity class draw function inside CurrentState Draw function like this:

GameStateFirstPhase::Draw(gfx); 

but i think that's illegal to do . I hope you get what i am trying to do and it makes sense . I also give image which i explain everything .

The question is if it possible to bind GameStateFirstPhase Draw function with a one of those states Draw functions (StartUp,SelectServer,ConnectSeerver state) bind i mean to get that parent behavior and add a child draw as well

Answer 1

"...bind i mean to get that parent behavior and add a child draw as well."

If you get around to asking about polymorphism, 'bind' is not the useable term. Research 'virtual', this is a modifier about a method.

No, the base class (parent) behavior (call it 'foo()') is not involved when a derived foo() is invoked. The derived foo() is not 'added' to the parent foo() in the sense of a sequence of method invocations.

However, the derived foo() can invoke the base foo(), at any point in its execution.

---

update - add small example base / derived / use of virtual

#include <iostream>
#include <vector>


 // base class
 class Foo
 {
 public:
    Foo() { std::cout << "Foo::Foo() " << std::endl; }

    virtual
    ~Foo() { std::cout << "Foo::~Foo() " << std::endl; }

    virtual void foo();
 };

 class Bar : public Foo
 {
 public:
    Bar() : Foo()
       { std::cout << "Bar::Bar() " << std::endl; }

    virtual
    ~Bar() { std::cout << "Bar::~Bar() " << std::endl; }

    virtual void foo();
 };

 void Foo::foo()  // virtual method
 {
    std::cout << " -- foo() in Foo " << std::endl;
 }

 void Bar::foo()  // also virtual method
 {
    Foo::foo();
    std::cout << " -- foo() in Bar " << std::endl;
    Foo::foo();
    std::cout << " -- foo() in Bar " << std::endl;
    Foo::foo();
 }


int testFoo(void)  // invoke from main
{
   std::cout << "\nsizeof(Foo): " << sizeof(Foo) << std::endl;
   std::cout << "sizeof(Foo*): " << sizeof(Foo*) << std::endl;

   std::cout << "\n";
   Foo* f = new Foo();   // ctor Foo one time

   std::cout << "\n";
   Bar* b = new Bar();   // ctor Foo + ctor Bar

   std::cout << "\n-----------------\n";
   f->foo();             // foo() in Foo

   std::cout << "\n";
   b->foo();             // foo() in xxx (5 lines)
   std::cout << "\n-----------------";

   // create a polymorphic vector
   std::vector<Foo*> fooVec;

   fooVec.push_back(f);  // Foo* added
   fooVec.push_back(b);  // Bar* added - accepted because "is-a" Foo
   // maybe 100's of different derived methods, added in arbitrary order

   for (auto x : fooVec )  // invoke foo() on ALL elements
   {
      std::cout << "\n";
      x->foo();
   }

   return(0);
}

output

sizeof(Foo): 8
sizeof(Foo*): 8

Foo::Foo() 

Foo::Foo() 
Bar::Bar() 

----------------- 
 -- foo() in Foo 

 -- foo() in Foo 
 -- foo() in Bar 
 -- foo() in Foo 
 -- foo() in Bar 
 -- foo() in Foo 

-----------------
 -- foo() in Foo 

 -- foo() in Foo 
 -- foo() in Bar 
 -- foo() in Foo 
 -- foo() in Bar 
 -- foo() in Foo 

Sorry this is so trivial ... my hope is that it helps lead you into research of virtual, pure virtual, etc.