C language: Struct Sub-typing

Thursday, 23/03/2023

Tram Ho

When using struct to describe simulated data records about entities, there are certain cases when between entities there is inheritance of properties, obviously we also want to reflect the inherited characteristic. inheritance in code instead of composition combination. inheritance is not syntactically supported by C , but technically by default this feature has been implemented for struct when working through struct pointer .

Structure Pointer

When working with struct through reference pointers, we always have the address of struct which is the address of the first data field in the definition of that struct type.

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

This means that if we store the address of a struct_person as the first data field of another struct type, the newly defined struct type can be considered an inheritor of class Person . Because if we have a Worker pointer that inherits from Person in this way, a function defined with an input parameter of Person will also work fine if we pass in the address of the first cell to be stored. Store the type 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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C language: Struct Sub-typing

Structure Pointer

Backward Casting

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