Cocos2dx Style and Implementation - c++

Question

I'm sort of getting back into cocos2dx development. This time I would like to fully understand the subtleties of the library. I have begun to get a good understanding of singleton classes, which they use throughout. I do have a question about their implementation. Here is a basic example of a singleton class being used
class GlobalClass
{
int m_value;
public:
GlobalClass(int v = 0)
{
m_value = v;
}
int get_value()
{
return m_value;
}
void set_value(int v)
{
m_value = v;
}
};
// Default initialization
GlobalClass *global_ptr = 0;
void foo(void)
{
// Initialization on first use
if (!global_ptr)
global_ptr = new GlobalClass;
global_ptr->set_value(1);
cout << "foo: global_ptr is " << global_ptr->get_value() << '\n';
}
void bar(void)
{
if (!global_ptr)
global_ptr = new GlobalClass;
global_ptr->set_value(2);
cout << "bar: global_ptr is " << global_ptr->get_value() << '\n';
}
int main()
{
if (!global_ptr)
global_ptr = new GlobalClass;
cout << "main: global_ptr is " << global_ptr->get_value() << '\n';
foo();
bar();
}
My question is about the initialization that comes after class definition and before foo, bar, and main. Essentially, I am interested in this line
GlobalClass *global_ptr = 0;
Where is the stack instance being initialized? You need some interface for that global pointer to the heap that is the member of that stack instance, right? If so, where is this done in cocos2dx?
My other question is about the use of the scope resolution operator (::) to call methods such as in
glview = GLViewImpl::create("MyView")
What purpose does this serve? Is this to overstep name spacing to access singleton instances?
If any of my understandings are wrong, especially on singletons, please correct them.

Answer 1

I think Syntax of your Singleton class is wrong.
It should be like this :
class GlobalClass
{
static GlobalClass* _instance; // Make private so only can access with method to avoid NPE for first time access
GlobalClass(int v = 0) // Make constructor private, you can't create object outside the class
{
m_value = v;
}
int m_value;
public:
static GlobalClass* getInstance();
int get_value()
{
return m_value;
}
void set_value(int v)
{
m_value = v;
}
};
GlobalClass* GlobalClass::_instance = 0;
GlobalClass* GlobalClass::getInstance() {
return GlobalClass::_instance == nullptr ? GlobalClass::_instance = new GlobalClass : GlobalClass::_instance;
}
void foo(void)
{
auto global_ptr = GlobalClass::getInstance();
global_ptr->set_value(1);
std::cout << "foo: global_ptr is " << global_ptr->get_value() << '\n';
}
void bar(void)
{
auto global_ptr = GlobalClass::getInstance();
global_ptr->set_value(2);
std::cout << "bar: global_ptr is " << global_ptr->get_value() << '\n';
}
int main()
{
auto global_ptr = GlobalClass::getInstance();
std::cout << "main: global_ptr is " << global_ptr->get_value() << '\n';
foo();
bar();
system("pause");
return 0;
}
And your second question :
Why we use or having static method create() almost in all class of cocos2d-x
In cocos2d-x we create object in two phase
- Create Object // No game Logic only set default_values
- Initialize Object
We're using Reference Count mechanism for memory management in cocos2d-x. what is it ?
These 2-phase constructor and auto-released reference count together into a static function: create()