OOP in C - Before GObject (Chinese Version)

OOP in C Before GObject [email_address] elpam@Taiwan, Taipei, 2008

LICENSE
本投影片授權方式為：
姓名標示─非商業性─相同方式分享 3.0 台灣版
http://creativecommons.org/licenses/by-nc-sa/3.0/tw/
所有的範例程式皆為 Public Domain

About This Slides
All example was build by
GCC-4.1.3
GLIB-1.2.10
GMAKE-3.81

Agenda
Constructor Review in C++
Constructor in C

C++ Language Review
Constructor
parent's constructor
member value initialize
initial member function (x)
do something
class Dog: public Animal{ private : string name; public: Dog(); }; Dog::Dog( const string n) : Animal() /*(1)*/ , name(n) /*(2)*/ { /* (4) */ cout << '' 汪汪 '' << endl; }

C++ Constructor
Which one?
cba
abc
/* 3-1.cpp */ class A{ public: A(){printf( “a” );}; }; class B: public A{ public: B(){printf( “b” );}; }; class C: public B{ public: C(){printf( “c” );}; }; C* c = new C;

C++ Constructor (cont')
Answer:
abc
Why parent's constructor execute first ?
在child的Constructor之中可能會Access到Parent的member value (及 member function)

C++ Default Constructor
Which one?
abc
abC
ABC
/* 3-2.cpp */ class A{ public: A(){printf( “a” );}; A( int ){printf( “A” );}; }; class B: public A{ public: B(){printf( “b” );}; B( int ){printf( “B” );}; }; class C: public B{ public: C(){printf( “c” );}; C( int ){printf( “C” );}; }; C* c = new C(1);

C++ Default Constructor (cont')
Answer:
abC
在沒有指定的情況之下Compiler會使用預設的Constructor
也就是沒有參數的那一個
如果將A:A()拿掉的話會發生什麼事?
/* 3-2.cpp */ class A{ public: A(){printf( “a” );}; A( int ){printf( “A” );}; }; class B: public A{ public: B(){printf( “b” );}; B( int ){printf( “B” );}; }; class C: public B{ public: C(){printf( “c” );}; C( int ){printf( “C” );}; }; C* c = new C(1);

Write Constructor Accurately constructor( Value ) : parent's constructor , member value initialize { /* do something */ } /* 3-3.hpp */ class A{ public: A(){ va = 0 ; }; A( int v){ va = v; }; private: int va; }; class B: public A{ public: B(); B( int ); private: int vb; }; /* ex3.cpp */ B::B() : A() , vb( 0 ) { printf( '' hello this is B() '' ); B::B( int v) : A( v ) /* !! */ , vb( v ) { printf( '' hello this is B(int) '' );

Constructor 該做的事情
do something
constructor( Value ) : parent's constructor , member value initialize { /* do something */ }

From C++ to C

Constructor in C
we use blahblah_new to replace malloc
命名方式並沒有語法上的限制
struct A{ int a; int b; }; struct A* A_new() { struct A* ret = malloc( sizeof (A) ); ret -> a = 0; ret -> a = 1; return ret; }

Constructor in C (cont')
Constructor
do something
struct A{ int a; }; struct B{ struct A parent; int a; }; struct A* A_new() { struct A* ret = malloc( sizeof (A) ); ret -> a = 0; return ret; } struct B* B_new() { struct B* ret = malloc( sizeof (B) ); ret -> parent -> a = 0; /* ???? */ ret -> b = 0; return ret; }

Constructor's Properties
不用宣告即有Virtual的屬性
執行繼承關係裡最下層的Constructor
Child的Constructor執行之前，Parent的Constructor 應該要先被執行
上面的範例需要手動的檢查，才能正確的執行parent's constructor

Constructor's Properties (cont')
Drawback: Parent's constructor can't be reused!
You need to rewrite the code what parent do
Initialize parent's member value
Initialize parent's member function!?
.............
Yes, what we think is the same. Boring jobs should be prevent. Stupid codes should be removed. Or we'll always find out stupid bug.
Let's why we use Gobject to do this

Conclusion
如果需要比較復雜的繼承關係，可以選擇GObject所提供的介面來完成
Gobject並不是使用C來完成C++/OOP裡全部的功能
我們將在下一個投影片之中導入GObject for OO

V-Table
Actually, above concept is coming from the implementation of V-Table (Virtual-Function-Table, Dispatch Table).
You can find out more information via reading OO Language's compiler implementation or some JIT interpreter's implementation.
http://en.wikipedia.org/wiki/Vtable

OOP in C - Before GObject (Chinese Version)

by Kai-Feng Chou on Nov 25, 2009

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