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POPointers to void pointers in C - can I use void** for rudimentary polymorphism?
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copied!<p>I can understand how a <code>void**</code> might look in memory, but I'm wondering if I'm using it quite right. Are there any fundamental flaws in what I describe below? For example, although I can say "it works for me", am I creating bad / unportable code in some way?</p> <p>So I have an Asteroids clone. There are three entities that can fire bullets, the players (<code>SHIP *player_1</code>, <code>SHIP *player_2</code>) and the UFO (<code>UFO *ufo</code>). When a bullet is fired, it's important to know who fired the bullet; if it was a player, when it hits something their score needs to be incremented. So, the bullet will store what kind of entity it belongs to (<code>owner_type</code>) and also a pointer directly to the owner (<code>owner</code>):</p> <pre><code>enum ShipType { SHIP_PLAYER, SHIP_UFO }; typedef struct Bullet { // ...other properties enum ShipType owner_type; void **owner; } BULLET; </code></pre> <p>Then, when the player hits the button or the UFO sees a target, one of these functions will be called:</p> <pre><code>void ship_fire(SHIP **shipp) { BULLET *bullet = calloc(1, sizeof(BULLET)); bullet->owner_type = SHIP_PLAYER; bullet->owner = (void**)shipp; // do other things } void ufo_fire(UFO **ufop) { BULLET *bullet = calloc(1, sizeof(BULLET)); bullet->owner_type = SHIP_UFO; bullet->owner = (void**)ufop; // do other things } </code></pre> <p>... they may be called, for example, like this:</p> <pre><code>ship_fire(&player_1); </code></pre> <p>Finally, when the bullet hits a target (such as an asteroid), we dereference the owner. If it's a ship, we can increment the score there and then.</p> <pre><code>void hit_asteroid(ASTEROID *ast, BULLET *bullet) { SHIP *ship_owner; if (bullet->owner_type == SHIP_PLAYER && *bullet->owner != NULL) { ship_owner = (SHIP*)*bullet->owner; ship_owner->score += 1000; } } </code></pre> <p>Does that seem a reasonable approach? Like I say, it works for me, but I only have a couple of months of C experience.</p> <p>A final note: why do I not use a <code>void*</code> instead of a <code>void**</code>? Because I want to avoid dangling pointers. In other words, say that <code>player_1</code> dies and is free'd, but their bullet keeps going and hits an asteroid. If I only have a <code>void*</code>, the <code>hit_asteroid</code> function has no way of knowing that <code>bullet->owner</code> points to de-allocated memory. But with a <code>void**</code>, I can validly check to see if it's NULL; if <code>player_1</code> is NULL, then <code>*bullet->owner</code> will be NULL too.</p> <p><strong>EDIT:</strong> All respondents so far concur that using a void** probably isn't necessary here because I can avoid the dangling pointers issue (by just statically allocating the base object, for instance). They're correct and I will refactor. But I'm still kinda interested to know if I've used void** in a way that might break something e.g. in terms of memory allocation / casting. But I guess if no-one has thrown their hands in the air and declared it faulty, it at least resembles something that would technically work.</p> <p>Thanks!</p>

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