Type casting in C++ confuses me to no end, but what confuses me even more is the traditional typecasting expression. Consider this
member (cricket_team);
Now, when I see that line out of context, I can’t make out if that is a function call or type-casting. Which makes me wonder why would someone use that when it can me done like this
(member) cricket_team;
I have a 13-inch MacBook Pro (7.1) with 250GB hard disk. And while installing Windows 7 to dual boot with OS X, I realized that I don’t have enough space for storage and installing applications. I had no choice but to upgrade. It is a good choice for two reasons, one I will be out of space before I realize and by then I would have invested considerable amount of time installing and customizing both OSes. Two, I can use the extracted disk for backup and Time Machine on OS X. With this in mind I order a 500GB (same brand and speed as already installed) hard disk drive (hdd) and a disk enclosure.
My order arrived on time, but little did I realize that I am missing two most important things to begin my work. Those were two types of screw drivers, Phillips #00 to open the back panel (pic. 4) and Torx 6 for removing the mounting screws (pic. 9) on the extracted disk to be put on the new disk (pic. 1 and 2). I read the MacBook Pro handout (pic. 3, the white booklet) on replacing hard disk, but it did not mention those screw drivers.
So, here is what I did to make the overall process of moving from old drive to a new one easy and safe. I started with noting down all the applications installed that are used frequently. Then exported all my calendar events, address book entries and bookmarks among other things to be imported on the new setup. Backed up important data, like pictures, videos and documents. Once this was done I was ready to put the new hdd. I followed the procedure for replacing hdd mentioned in the MacBook Pro booklet provided. The instructions in the booklet are clear, and the pictures below of my endeavor should make it even more lucid. Hence, I am skipping the details of installing it. Only word of caution is to be careful of static and not touching hdd on the top (to avoid harming head and disk) and bottom (to avoid touching the circuitry). Make sure that it is always handled on sides (as in pic. 9 and 10). General idea while handling electronic stuff is to read the cautionary instructions on the device.
Finally, I installed OS X. Though there was a hick up of installer not detecting hdd, besides everything else going fine. I resolved the hdd issue by running disk utility from utilities and creating one partition and formatting it. I also ran a sanity check on the hdd, to be sure. Furthermore, I also noticed that Windows 7 installation on MacBook Pro using bootcamp doesn’t finish in one go. It hung the first time and went fine the second time.
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(I am sticking with Windows Live writer, as I couldn’t find a free and better alternative.)
If there is C++, what is the need of doing Object Oriented Programming (OOP) in C? Well I don’t have a compelling reason other than that it is a fun exercise. So, with that in mind, I am going to show an approach to OOP in C next. It is as close to OOP but not exactly OOP - for one it is not compiler restricted programming - that is compiler won’t crib if I break the OOP rules - and two, this implementation is short on some parameters of OOP. Which I will examine as I go along the example.
Note: The programmer should understand the underlying assumptions and should enforce the rules of programming to achieve OOP in C, as outlined below.
I will use two main features of C to attain encapsulation and function polymorphism. Namely, function pointers and static functions. Accompanied by segregation of public and private members in header files.
I start with a public header file –publicclass.h- of the class, which should be included wherever that class is needed.
| typedef struct _testClass testClass; struct _testClass { int publicData; void (*publicFunc)(testClass*); }; // function to create testClass testClass *constructClass(void); void destructClass(testClass*); |
The structure testClass above defines public members of the class. Every function declared has to take a pointer to the structure of which it is a member. This is done to emulate 'this', as in C++. So, that we know the object being referred to. Also constructor and a destructor functions are defined, which create and destroy the class object respectively. These are like C++ constructors/destructors in a loose sense. These have to be called the first (for constructor) and the last (for destructor) in the object life cycle. The public function pointer defined will be discussed later.
Next I declare a private header file –privateclass.h- , to encapsulate the private data of the class. This is done by exclusively including this file in the class declaration file – class.c- and not anywhere else. This is one of the user coding restrictions to enforce data encapsulation.
| #include "publicclass.h" typedef struct _itestClass itestClass; struct _itestClass { // public: testClass public; // private: int privateData; void (*privateFunc)(itestClass*); }; |
The structure above is called itestClass, where ‘i’ stands for internal or more appropriately private (I avoided using ‘p’ for private as to not confuse it with pointer). This structure looks similar to the previous one but with one difference, that is, it includes the public ‘testClass’ structure on the top - it is the first member. This serves a simple purpose of simplifying the pointer arithmetic, such that the pointer to ‘public’ (of ‘testClass’) will be the pointer to ‘itestClass’. This will become clear on the actual class implementation, below. Note, that the private functions takes pointer of ‘itestClass’.
Finally, the class implementation –class.c-. It is more or less self explanatory, where both public and private functions are defined. And in constructor assigned to the created object, and the pointer to the public structure –testClass- returned.
| #include "publicclass.h" #include "privateclass.h" static void publicFunc(testClass *this) { itestClass *ithis = (itestClass*)this; printf("This is publicFunc\n"); this->publicData = 20; printf("From publicFunc: publicData: %d\n", this->publicData); ithis->privateData = 30; printf("From publicFunc: privateData: %d\n\n", ithis->privateData); ithis->privateFunc(ithis); } static void privateFunc(itestClass *ithis) { printf("This is privateFunc\n"); } testClass *constructClass(void) { itestClass *pobj = (itestClass*)calloc(1, sizeof(itestClass)); pobj->public.publicData = 0; pobj->public.publicFunc = publicFunc; pobj->privateData = 0; pobj->privateFunc = privateFunc; return &pobj->public; } void destructClass(testClass *pobj){ if (pobj) { free(pobj); } } |
As mentioned earlier ‘this’ (pointer to class object) is used to get ‘ithis’ (pointer to internal class object). In a class declaration all the functions are ‘static’ (except the constructor and destructor), to avoid namespace pollution and achieve function polymorphism. Such that, another class can be defined with a different implementation of the functions. Apart from function polymorphism class hierarchy can also be achieved and that is left as an exercise for the reader :-). Note: only this file includes the private header file –privateclass.h.
And lastly here is how to use the above built OOP structure in C.
| #include "publicclass.h" int main (int argc, const char * argv[]) { testClass *pobj = constructClass(); pobj->publicData = 10; printf("From main: publicData: %d\n\n", pobj->publicData); // Error: not a member of obj(testClass) and itestClass is not visible // pobj->privateData = 20; pobj->publicFunc(pobj); destructClass(pobj); return 0; } |
Output.
| From main: publicData: 10 This is publicFunc From publicFunc: publicData: 20 From publicFunc: privateData: 30 This is privateFunc |
Download this example from here.
(I am no longer using line numbers to post the code. I realized that with line numbers it is difficult to make changes in the post. On a change I have to update line numbers, else leave it discontinuous.)
