'OOP'에 해당되는 글 3건

  1. 2011.05.17 [C/C++] Tips for OOP - module initialization.
  2. 2010.11.12 [C/C++] Encapsulation tip in C.
  3. 2010.04.05 [Prog] Object Oriented Programming

[C/C++] Tips for OOP - module initialization.

Language/C&C++ 2011.05.17 17:00

OOP SW design is usually compose of one main control routine (henceforth MCR) and lots of sub modules.
But, MCR don't need to (must not need to) know inside implementation of every sub modules.
Sometimes, MCR don't need to know even existence of some sub modules.
The problem is, most sub modules require initialization.
How can sub modules whose existence is now known, be initialized.
Due to this issue, principle of information hiding is sometimes broken.
Let's see below example.

FILE : main.c
-------------
int main(int argc, char* argv[]) {
        ...

}

FILE : moduleA.c
----------------
...

FILE : moduleB.c
----------------
...

Assume that, each module requires initialization and main.c don't need to know existence of each module.
How can we resolve this issue?
Easiest way is calling initialization function of each module with damaging principle of information hiding little bit, like mentioned above.

FILE : main.c
-------------
extern void moduleA_init();
extern void moduleB_init();

int main(int argc, char* argv[]) {
        ...
        moduleA_init();
        moduleB_init();
        ...
}

FILE : moduleA.c
----------------
...
void moduleA_init() { ... }

FILE : moduleB.c
----------------
...
void moduleB_init() { ... }

At above code, main.c becomes to know existence of moduleA and moduleB.
That is, in terms of modules, principle of information hiding is damaged although it's very little.
Additionally, global symbol space is dirtier.
Regarding maintenance, whenever new module is added, modifying main.c is unavoidable.
But, main.c doesn't have any dependency on newly added module.
With this and that, this way is uncomfortable.
How can we clean up these?
Using constructor leads us to better way.

Functions in constructor are executed before main function.
So, it is very useful for this case.
Easiest way is setting every initialization function as constructor.
But, in this case, we cannot control the moment when module is initialized at.
Therefore, it is better that each module's initialization function is registered to MCR, and MCR calls these registered function at right moment.
Following pseudo code is simple implementation of this concept.

FILE : main.c
-------------
void register_initfn(void (*fn)()) {
        list_add(initfn_list, fn);
}

int main(int argc, char* argv[]) {
        ...
        /* initialize modules */
        foreach(initfn_list, fn)
                (*fn)();
        ...
}

FILE : module.h
---------------
extern void register_initfn(void (*fn)());
#define MODULE_INITFN(fn)                               \
        static void __##fn##__() __attribute__ ((constructor)); \
        static void __##fn##__() { register_initfn(&fn); }

FILE : moduleA.c
----------------
...
#include "module.h"
...
static void _myinit() { ... }
MODULE_INITFN(_myinit)

FILE : moduleB.c
----------------
...
#include "module.h"
...
static void _myinit() { ... }
MODULE_INITFN(_myinit)

Now, MCR don't need to know existence of each modules.
And, MCR can also control the moment of each module's initialization.
In addition, adding new module doesn't require any modification of MCR side.
It is closer to OOP's concept, isn't it?

We can improve this concept by customizing memory section.
Here is rough description of this.

* Declare special memory section for initializer function.
    - In gcc, ld script should be modified.

* Put initializer function into this section.
    - __attribute__ ((__section__("xxxx"))) can be used.

* MCR can read this section and call these functions at appropriate moment.

Actually, this way is better than using constructor in terms of SW design.
Linux kernel uses this concept in it's driver model.
(For deeper analysis, kernel source code can be good reference.)
But, in many cases, using this concept may lead to over-engineering.
So, if there isn't any other concern, using constructor is enough.

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[C/C++] Encapsulation tip in C.

Language/C&C++ 2010.11.12 17:16

Usually, pointer of not-opened-structure is used to hide module's information.
C dummies may use 'void*' to do this. But, it's not good way.
Let's see following example.
(Following codes are test in GCC4.4 with '-Wall' option.

typedef void module_t;             /* <-- *1 */
typedef struct _sModule module_t;  /* <-- *2 */

module_t* create_module(int arg);
int       do_something(module_t* m, int arg);
...
do_something((int*)m, 1); /* <-- *a */

Pointer of any type can be casted to 'void*', and 'void*' can be casted to pointer of any type without warning.
So, in case of (*1), (*a) doesn't generate any warning. That is, it is NOT TYPE SAFE (in compile time).
But, in case of (*2), GCC give warning like "... incompatible pointer type ...". It's TYPE SAFE.
And, interestingly, compiler doesn't complain anything about (*2) because, compiler doesn't need to know size of 'struct _sModule'.
Only pointer is used. So, knowing size of pointer type is enough and compiler already know it.
So, in terms of syntax, it's ok too!

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tags : OOP
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[Prog] Object Oriented Programming

Essay/Software 2010.04.05 17:40

The most important thing of OOP is "Making objects that developer don't need to look inside of it.". Reliable objects are foundation of OOP. Then let's think about this more.

* Object should respond at all cases.

That is, crashing inside object should not be happened! object should be in valid state at any case. If software is crashed inside object, developer should analyze internal code of object, and this is what we want to avoid.

* Reducing possibility of misuse interface should be considered when interface is design.

That is, usage of interfaces should be easy and intuitive. If not, developer need to investigate for object's internal parts to know correct usage. And, usually, interdependent interfaces leads to misuse. For example, "Interface B should be used after interface A is called", "Interface A should not be called after interface B is called" and so on. So, if possible, reduce dependency among interfaces of objects.

* Interface should be well-commented.

In the same context with above, developer don't need to look into the object that is well-commented. In my opinion, comments of interface should include at least following things.
  + behavior of interface.
  + prerequisite to use the interface.
  + explanation about each parameters.
  + description about return value.
And, adding usage example is recommended.

* For object to starts supporting minimal set of interface is better.

Adding interface is easy. But deleting interface is difficult because it may affect to number of other objects that already use the interface. And, stabilizing object having small number of interface, is easier. Starting from minimal set can also help developer escape from temptation of over-engineering

 
Next important point is relations among objects.
Software created by OOP consists of lots of objects and relations. To understand software's behavior, knowing relations and inter-operations among objects is significant.

* Software documentation.

Design concept, policy, block diagram, used patterns and so on is needed to be documented to help reader to understand overall shape of software.

 
There is also disadvantage of OOP.

* Performance drop.

In every object, there are lots of routines for checking and handling unexpected cases. And in general, software in OOP consists of lots of objects. So, inevitably, number of duplicated check and handling are unavoidable - all objects may check same thing. And sometimes this drops performance very much.

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