Ngôn ngữ C: Mở Ruộng Struct

Thứ Năm, 23/03/2023

Tram Ho

Khi sử dụng struct để mô tả các bản ghi dữ liệu mô phỏng về các thực thể, có những trường hợp nhất định khi giữa các thực thể có tính chất kế thừa các đặc tính, hiển nhiên chúng ta cũng muốn phản ánh đặc trưng kế thừa inheritance trong code thay vì tổ hợp composition. Tính năng kế thừa inheritance không được C hỗ trợ biểu thị về mặt cú pháp, tuy nhiên về mặt kĩ thuật thì mặc định là tính năng này đã được triển khai đối với các struct khi làm việc thông qua các con trỏ struct pointer.

Struct Pointer

Khi làm việc với các struct thông qua con trỏ tham chiếu, chúng ta luôn có địa chỉ của struct chính là địa chỉ của trường dữ liệu đầu tiên trong định nghĩa của kiểu struct đó.

typedef struct {
   char* class;
   char* name;
   int   age;
} person_struct;

person_struct person (char* $name, int $age);


typedef person_struct* Person;

Person new_Person (char* $name, int $age);

typedef struct {

char* class;

char* name;

int age;

} person_struct;

person_struct person (char* $name, int $age);

typedef person_struct* Person;

Person new_Person (char* $name, int $age);

<span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string">"Person.h"</span></span>


person_struct <span class="token function">person</span>  <span class="token punctuation">(</span>
   <span class="token keyword">char</span><span class="token operator">*</span> $name<span class="token punctuation">,</span> 
   <span class="token keyword">int</span>   $age
<span class="token punctuation">)</span> <span class="token punctuation">{</span>
   person_struct $primitive <span class="token operator">=</span> <span class="token punctuation">{</span>
      <span class="token punctuation">.</span>class <span class="token operator">=</span> <span class="token string">"Person"</span><span class="token punctuation">,</span>
      <span class="token punctuation">.</span>name  <span class="token operator">=</span> $name<span class="token punctuation">,</span>
      <span class="token punctuation">.</span>age   <span class="token operator">=</span> $age
   <span class="token punctuation">}</span><span class="token punctuation">;</span>
   <span class="token keyword">return</span> $primitive<span class="token punctuation">;</span>
<span class="token punctuation">}</span>


Person <span class="token function">new_Person</span> <span class="token punctuation">(</span>
   <span class="token keyword">char</span><span class="token operator">*</span> $name<span class="token punctuation">,</span> 
   <span class="token keyword">int</span>   $age
<span class="token punctuation">)</span> <span class="token punctuation">{</span>
   person_struct $primitive <span class="token operator">=</span> <span class="token function">person</span> <span class="token punctuation">(</span>$name<span class="token punctuation">,</span> $age<span class="token punctuation">)</span><span class="token punctuation">;</span>
   Person $reference <span class="token operator">=</span> <span class="token operator">&amp;</span> <span class="token punctuation">(</span>$primitive<span class="token punctuation">)</span><span class="token punctuation">;</span>
   <span class="token keyword">return</span> $reference<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

#include "Person.h"

person_struct person (

char* $name,

int $age

) {

person_struct $primitive = {

.class = "Person",

.name = $name,

.age = $age

};

return $primitive;

}

Person new_Person (

char* $name,

int $age

) {

person_struct $primitive = person ($name, $age);

Person $reference = & ($primitive);

return $reference;

}

<span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string">&lt;stdio.h&gt;</span></span>
<span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string">"Person.h"</span></span>

<span class="token keyword">void</span> <span class="token function">main</span> <span class="token punctuation">(</span>
   <span class="token keyword">int</span>   $argc<span class="token punctuation">,</span>
   <span class="token keyword">char</span><span class="token operator">*</span> $argv<span class="token punctuation">[</span><span class="token punctuation">]</span>
<span class="token punctuation">)</span> <span class="token punctuation">{</span>
   Person $me <span class="token operator">=</span> <span class="token function">new_Person</span> <span class="token punctuation">(</span><span class="token string">"Semi Dev_"</span><span class="token punctuation">,</span> <span class="token number">1001</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
   <span class="token function">printf</span> <span class="token punctuation">(</span><span class="token string">"Class: %s"</span><span class="token punctuation">,</span> $me<span class="token operator">-&gt;</span>class<span class="token punctuation">)</span><span class="token punctuation">;</span>
   <span class="token function">printf</span> <span class="token punctuation">(</span><span class="token string">"References: n"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
   <span class="token function">printf</span> <span class="token punctuation">(</span><span class="token string">"   struct: %p"</span><span class="token punctuation">,</span> $me<span class="token punctuation">)</span><span class="token punctuation">;</span>
   <span class="token function">printf</span> <span class="token punctuation">(</span><span class="token string">"   .class: %p"</span><span class="token punctuation">,</span> <span class="token operator">&amp;</span><span class="token punctuation">(</span>$me<span class="token operator">-&gt;</span>class<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

#include <stdio.h>

#include "Person.h"

void main (

int $argc,

char* $argv[]

) {

Person $me = new_Person ("Semi Dev_", 1001);

printf ("Class: %s", $me->class);

printf ("References: n");

printf (" struct: %p", $me);

printf (" .class: %p", &($me->class));

}

Class: Person
References:
   struct: 0x7ffd01db6890
   .class: 0x7ffd01db6890

Class: Person

References:

struct: 0x7ffd01db6890

.class: 0x7ffd01db6890

Backward Casting

Điều này có nghĩa là nếu như chúng ta lưu địa chỉ của một struct_person ở vị trí trường dữ liệu đầu tiên của một kiểu struct khác thì kiểu struct mới định nghĩa sẽ có thể được xem là một kiểu kế thừa từ class Person. Bởi giả sử nếu chúng ta đang có một con trỏ Worker kế thừa từ Person theo cách này, một hàm được định nghĩa có tham số đầu vào là Person cũng sẽ hoạt động tốt nếu chúng ta truyền vào địa chỉ của ô nhớ đầu tiên lưu trữ kiểu Worker.

#include "Person.h"


typedef struct {
   person_struct super;
   char*         class;
   int           stamina;
} worker_struct;

worker_struct worker (char* $name, int $age, int $stamina);


typedef worker_struct* Worker;

Worker new_Worker (char* $name, int $age, int $stamina);

#include "Person.h"

typedef struct {

person_struct super;

char* class;

int stamina;

} worker_struct;

worker_struct worker (char* $name, int $age, int $stamina);

typedef worker_struct* Worker;

Worker new_Worker (char* $name, int $age, int $stamina);

<span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string">"Worker.h"</span></span>


worker_struct <span class="token function">worker</span> <span class="token punctuation">(</span>
   <span class="token keyword">char</span><span class="token operator">*</span> $name<span class="token punctuation">,</span> 
   <span class="token keyword">int</span>   $age<span class="token punctuation">,</span> 
   <span class="token keyword">int</span>   $stamina
<span class="token punctuation">)</span> <span class="token punctuation">{</span>
   person_struct $super <span class="token operator">=</span> <span class="token function">person</span> <span class="token punctuation">(</span>$name<span class="token punctuation">,</span> $age<span class="token punctuation">)</span><span class="token punctuation">;</span>
   worker_struct $primitive <span class="token operator">=</span> <span class="token punctuation">{</span>
      <span class="token punctuation">.</span>super   <span class="token operator">=</span> $super<span class="token punctuation">,</span>
      <span class="token punctuation">.</span>class   <span class="token operator">=</span> <span class="token string">"Worker"</span><span class="token punctuation">,</span>
      <span class="token punctuation">.</span>stamina <span class="token operator">=</span> $stamina 
   <span class="token punctuation">}</span><span class="token punctuation">;</span>
   <span class="token keyword">return</span> $primitive<span class="token punctuation">;</span>
<span class="token punctuation">}</span>


Worker <span class="token function">new_Worker</span> <span class="token punctuation">(</span>
   <span class="token keyword">char</span><span class="token operator">*</span> $name<span class="token punctuation">,</span> 
   <span class="token keyword">int</span>   $age<span class="token punctuation">,</span> 
   <span class="token keyword">int</span>   $stamina
<span class="token punctuation">)</span> <span class="token punctuation">{</span>
   worker_struct $primitive <span class="token operator">=</span> <span class="token function">worker</span> <span class="token punctuation">(</span>$name<span class="token punctuation">,</span> $age<span class="token punctuation">,</span> $stamina<span class="token punctuation">)</span><span class="token punctuation">;</span>
   Worker $reference <span class="token operator">=</span> <span class="token operator">&amp;</span> <span class="token punctuation">(</span>$primitive<span class="token punctuation">)</span><span class="token punctuation">;</span>
   <span class="token keyword">return</span> $reference<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

#include "Worker.h"

worker_struct worker (

char* $name,

int $age,

int $stamina

) {

person_struct $super = person ($name, $age);

worker_struct $primitive = {

.super = $super,

.class = "Worker",

.stamina = $stamina

};

return $primitive;

}

Worker new_Worker (

char* $name,

int $age,

int $stamina

) {

worker_struct $primitive = worker ($name, $age, $stamina);

Worker $reference = & ($primitive);

return $reference;

}

<span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string">&lt;stdio.h&gt;</span></span>
<span class="token macro property"><span class="token directive-hash">#</span><span class="token directive keyword">include</span> <span class="token string">"Worker.h"</span></span>


<span class="token keyword">void</span> <span class="token function">put_person</span> <span class="token punctuation">(</span>Person $person<span class="token punctuation">)</span> <span class="token punctuation">{</span>
   <span class="token function">printf</span> <span class="token punctuation">(</span><span class="token string">"Class : %s n"</span><span class="token punctuation">,</span> $person<span class="token operator">-&gt;</span>class<span class="token punctuation">)</span><span class="token punctuation">;</span>
   <span class="token function">printf</span> <span class="token punctuation">(</span><span class="token string">"Name  : %s n"</span><span class="token punctuation">,</span> $person<span class="token operator">-&gt;</span>name<span class="token punctuation">)</span><span class="token punctuation">;</span>
   <span class="token function">printf</span> <span class="token punctuation">(</span><span class="token string">"Age   : %i n"</span><span class="token punctuation">,</span> $person<span class="token operator">-&gt;</span>age<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>


<span class="token keyword">void</span> <span class="token function">main</span> <span class="token punctuation">(</span>
   <span class="token keyword">int</span>   $argc<span class="token punctuation">,</span>
   <span class="token keyword">char</span><span class="token operator">*</span> $argv<span class="token punctuation">[</span><span class="token punctuation">]</span>
<span class="token punctuation">)</span> <span class="token punctuation">{</span>
   Worker $me <span class="token operator">=</span> <span class="token function">new_Worker</span> <span class="token punctuation">(</span><span class="token string">"Semi Dev_"</span><span class="token punctuation">,</span> <span class="token number">1001</span><span class="token punctuation">,</span> <span class="token number">10000</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
   <span class="token function">put_person</span> <span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token keyword">void</span><span class="token operator">*</span><span class="token punctuation">)</span> $me<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

#include <stdio.h>

#include "Worker.h"

void put_person (Person $person) {

printf ("Class : %s n", $person->class);

printf ("Name : %s n", $person->name);

printf ("Age : %i n", $person->age);

}

void main (

int $argc,

char* $argv[]

) {

Worker $me = new_Worker ("Semi Dev_", 1001, 10000);

put_person ((void*) $me);

}

Class : Person 
Name  : Semi Dev_ 
Age   : 1001

Class : Person

Name : Semi Dev_

Age : 1001

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Ngôn ngữ C: Mở Ruộng Struct

Struct Pointer

Backward Casting

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