Question

If i have a class name app and a class name manager. I want to create inside manager class an app class, I’ve seen two options but I didn’t understand the difference.

choice 1:

//.....
class app;  //writing this line outside the declaration of the func
class manager{
//..   
private:
 app *a;
//...
}

or choice 2:

class manager{
//.. 
private:
app *a;
//..
}

Answer 1

The class app; outside your class manager { ... } is a forward declaration. It tells the compiler, “app is a class. I’m not going to tell you what it looks like, but trust me, it is.”

Think of your compiler as only knowing about this particular file (which I am assuming is a .h file) when it compiles. You have said that your member class includes a pointer to an “app“. “What the heck is an app?” the compiler wants to know. Without the forward declaration (or perhaps an #include app.h or something similar), it doesn’t know, and will fail to compile. By saying at the top, class app;, it knows that app is a class. That’s all it needs to know to allocate the space for a pointer to it in your member class.