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.)
It took me a while to figure out how to publish C/C++ code to the blog(spot). While doing so I messed up the post a few times. Finally, I got it working, and the way I wanted it, such that it looks same across the browsers and is well formatted. I tried a few things and some of them didn’t work. What didn’t work for me were plugins in Windows Live Writer (LW) to post code. Some of them didn’t show the colored text for the keywords and some didn’t have uniformity across browsers.
What worked was gvim/vim. Vim provides a command ‘TOhtml’ to generate html for the code with proper syntax – exactly the way seen in vim. But that is half the job, as BlogSpot doesn’t take the html code with <head> and <body> tags of the generated html. To get it right I had to do two things. First configure vimrc to generate the desired syntax and scheme (more on this later), and then copying the right part from the generated html. Here I will write about the way I did it using Live Writer (LW).
The published code in previous posts is inside a table, which is easy to draw in LW. So, first draw a table (1x1) in edit mode. Then in source mode copy the html generated by vim, starting from first <font> tag to the closing of it (which is at the end of the file) into that table. Even without LW this is easy to do, just add 1x1 table tags in BlogSpot edit post mode and then paste the generated html code. Explained below.
Copy the vim generated html code (as explained above) between these <td> tags.
<td valign="top" width="625"> {copy here} </td>
Note: Remove <pre> tags if any between the <td> tags. width can be adjusted to fit the blog.
After this add bgcolor of the <body> tag from the generated html to the <table> tag in the html code of the post. This will give the background color similar to vim scheme. At this point we have everything in place except the part, that this post won’t look same across the browsers. The reason being, first <font> tag in the pasted code. Where face is set as ‘monospace’ and not a particular fixed width font, like ‘Courier’. Hence, replace whatever there is with this ‘color="#ffffff" size="2" face="Consolas"’. This also gives the color to the ‘regular text’ in the code. In this case white (#ffffff).
This is how it will looked like on completion. Copied code marked in green.
<table border="0" cellspacing="0" cellpadding="3" width="625" bgcolor="#333333">
<tbody> <tr> <td valign="top" width="625">
<font color="#ffffff" size="2" face="Consolas, Courier New">
<Rest of the generated html code>
</font>
</td> </tr> </tbody>
</table>
| 1 int main(int argc, char * const argv[]) { 2 return 0; 3 } |
Here is the vimrc configuration used to get the syntax, scheme and the format. Just add these lines to the existing text. Line numbering can be turned off. If vimrc file can’t be located, typing in command - ‘e $MYVIMRC’ – will fetch it. html_use_css, is a switch to enable/disable CSS in the generated html code. The example above is without CSS styles.
set nu
colorscheme desert
syntax on
set guifont=Courier_New:h10
let html_use_css=0
(After writing this post, I am thinking of doing away with LW. I had hard time formatting and getting the post as I wanted.)
This code is to explain how polymorphism works through virtual functions and how C++ does it using VPTR and VTABLE. Please refer to my last post - C++ test code on virtual functions – to understand this better.
| Code example to find out VPTR and VTABLE in C++ class. And use it to show how polymorphism works in C++. That is same base class pointer can be used to invoke functions of the derived class. C++ achieves this by changing the VPTR pointer to new drived classes VTABLE. VPTR and VTABLE usage for polymorphism in C++ Class expalined diagrammatically below. The base class object VPTR changes when base class is instantiated with that sub class, to point to the VTABLE of that sub class. '?' - denotes VPTR will decide what subclass is currently instantiated to the base class. BaseClass Object (pobj) SubClass_1 | VTABLE (vtable) v | +------------+ v | VPTR (vptr)| *-->+------------+ +------------+ ? | VTEntry1 |------------> +------------+ | DATA1 (a) | * +------------+ | func1() | +------------+ | | VTEntry2 |--------+ | | | DATA2 (b) | | +------------+ | | | +------------+ | | . | | +------------+ | . | | | . | | | . | | | . | +---->+------------+ | . | | | func2() | | . | | | | | | | | +------------+ | SubClass_2 | VTABLE (vtable) | | | v +-->+------------+ | VTEntry1 |------------> +------------+ +------------+ | func1() | | VTEntry2 |--------+ | | +------------+ | | | | . | | +------------+ | . | | | . | +---->+------------+ | func2() | | | | | +------------+ |
| +------------------+ | BaseClass | +------------------+ / \ / \ / \ V V +------------------+ +------------------+ | SubClass_1 | | SubClass_2 | +------------------+ +------------------+ |
| typedef void (*func)(void); // class member function type typedef int* ptr; // 32 bit system pointer type class BaseClass { private: int a; // DATA1 public: BaseClass() : a(0) {} // pure virtual function virtual void func1() =0; virtual void func2() =0; static void deref_vtable(ptr); }; class SubClass_1 : public BaseClass { public: void func1() { cout << "SubClass_1::func1" << endl; } void func2() { cout << "SubClass_1::func2" << endl; } }; class SubClass_2 : public BaseClass { public: void func1() { cout << "SubClass_2::func1" << endl; } void func2() { cout << "SubClass_2::func2" << endl; } }; // static function to dereference the vtable entries void BaseClass::deref_vtable(ptr vtable) { func pfunc = NULL; // dereferencing first (VTABLE1) entry in VTABLE pfunc = (func)*(vtable+0); pfunc(); // dereferencing second (VTABLE2) entry in VTABLE pfunc = (func)*(vtable+1); pfunc(); return; } int main(int argc, char * const argv[]) { BaseClass *pobj = NULL; ptr vptr, vtable; // first sub class pobj = new SubClass_1; vptr = (ptr)pobj; // first address of obj (class) points to vptr vtable = (ptr)*vptr; // dereferencing vptr to get vtable cout << "vptr: " << vptr << " " << "vtable: " << vtable << endl; BaseClass::deref_vtable(vtable); delete pobj; cout << endl; // second sub class pobj = new SubClass_2; vptr = (ptr)pobj; // first address of obj (class) points to vptr vtable = (ptr)*vptr; // dereferencing vptr to get vtable cout << "vptr: " << vptr << " " << "vtable: " << vtable << endl; BaseClass::deref_vtable(vtable); delete pobj; return 0; } |
| vptr: 0x9934008 vtable: 0x8048cf8 SubClass_1::func1 SubClass_1::func2 vptr: 0x9934008 vtable: 0x8048ce8 SubClass_2::func1 SubClass_2::func2 |
I did this stuff while understanding virtual functions and polymorphism in C++ for job interviews. And got a chance to clean it up and comment it well (of course to be put on a blog).
This ones on virtual functions, explaining how VPTR and VTABLE work. Code is self explanatory, accompanied by a diagram.
| Code example to find out VPTR and VTABLE in C++ class. And use it to probe class virtual functions. VPTR and VTABLE usage in C++ Class expalined diagrammatically below. Test Class Object (obj) | VTABLE (vtable) v | +------------+ v | VPTR (vptr)|------>+------------+ +------------+ | VTEntry1 |------------> +------------+ | DATA1 (a) | +------------+ | func1() | +------------+ | VTEntry2 |-------+ | | | DATA2 | +------------+ | | | +------------+ | . | | +------------+ | . | | . | | | . | | . | +----->+------------+ | . | | func2() | | . | | | | | +------------+ |
| typedef void (*func)(void); // class member function type typedef int* ptr; // 32 bit system pointer type class Test { public: int a; // DATA1 Test() : a(0) {} virtual void func1() { cout << "Test::func1" << endl; } virtual void func2() { cout << "Test::func2" << endl; } }; int main(int argc, char * const argv[]) { Test obj; int *pdata; ptr vptr, vtable; func pfunc = NULL; vptr = (ptr)&obj; // first address of obj (class) points to vptr vtable = (ptr)*vptr; // dereferencing vptr to get vtable pdata = (int*)((ptr)&obj+1); // this is 'a', the first member of class Test *pdata = 10; cout << "pdata (a): " << *pdata << endl; cout << "vptr: " << vptr << " " << "vtable: " << vtable << endl; // dereferencing first (VTEntry1) entry in VTABLE pfunc = (func)*(vtable+0); pfunc(); // dereferencing second (VTEntry2) entry in VTABLE pfunc = (func)*(vtable+1); pfunc(); return 0; } |
| pdata (a): 10 vptr: 0xbfee67d4 vtable: 0x8048a80 Test::func1 Test::func2 |
