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POUsing pointer functions to emulate member functions in C structs
 

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    <p>So just for the sake if having ''fun'' I decided to emulate <code>C++</code> member functions in <code>C</code> using pointer functions. Here is a simple code:</p> <p>obj.h:</p> <pre><code>#ifndef OBJ_H #define OBJ_H #include &lt;stdlib.h&gt; #include &lt;stdio.h&gt; struct Obj{ struct pObjVar* pVar; void (*read)(struct Obj*); void (*set) (struct Obj*, int); }; struct Obj* newObj(); void deleteObj(struct Obj** obj); #endif </code></pre> <p>obj.c:</p> <pre><code>#include "obj.h" void readValue(struct Obj* this_); void setValue (struct Obj* this_, int mValue_); struct pObjVar{ int mValue; }; struct Obj* newObj(){ struct Obj* tmp = (struct Obj*) malloc(sizeof(struct Obj)); tmp-&gt;pVar = (struct pObjVar*) malloc(sizeof(struct pObjVar)); tmp-&gt;pVar-&gt;mValue = 0; tmp-&gt;read = readValue; tmp-&gt;set = setValue; return tmp; } void deleteObj(struct Obj **obj){ free((*obj)-&gt;pVar); (*obj)-&gt;pVar = NULL; free((*obj)); *obj = NULL; } void readValue(struct Obj *this_){ printf("Value = %d\n",this_-&gt;pVar-&gt;mValue); } void setValue(struct Obj *this_, int mValue_){ this_-&gt;pVar-&gt;mValue = mValue_; } </code></pre> <p>main.c:</p> <pre><code>#include "obj.h" int main(void) { struct Obj* a = newObj(); a-&gt;set(a, 10); a-&gt;read(a); deleteObj(&amp;a); return 0; } </code></pre> <p>Output:</p> <pre><code>&gt;./a.out Value = 10 </code></pre> <p>In doing this, however, I figured I had to emulate the role of implicit <code>this</code> pointer by explicitly passing it to my member functions. This works fine, I guess, except that it makes the whole thing look weird! </p> <p>If I wanted to pass the object, why would implement the functions as member functions? The only answer I found to it was maybe in cases where you would want to have a unified interface but various implementations? (something <em>similar</em> to <code>C++</code> virtual functions?) </p> <p>What are (if any) some other reasons to emulate member functions? Also, is there any way to get around passing the explicit <code>this_</code> pointer at all?</p> <p><strong>EDIT</strong>: There was problem in the original code when passing the object. I was using <code>&amp;a</code> by mistake for the <code>read/set</code> functions. You would only need it for the <code>deleteObj</code> if you want to set the pointer to <code>NULL</code> internally.</p>
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    1. COYou are passing the _address_ of the pointer to the functions, i.e. `Obj **`. Skip the `&` in the function calls.
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    2. CO@JoachimPileborg: You are correct. Please see my edit
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    3. COI once had to maintain a legacy project using a similar approach, with a complex inheritance hierarchy, and the added perversion of using opaque typedef'ed int values instead of pointers to structures. Needless to say that it was a nightmare to debug/maintain !
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