Generic linked list function

This is a discussion on Generic linked list function within the C Programming forums, part of the General Programming Boards category; I have three types of linked list in a program: Code: GFX_ENTRY* gfx_ls; GFX_ENTRY* gfx_le; TILEDEF* tile_ls; TILEDEF* tile_le; SPRITEDEF* ...

  1. #1
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    Generic linked list function

    I have three types of linked list in a program:
    Code:
    GFX_ENTRY* gfx_ls;
    GFX_ENTRY* gfx_le;
    TILEDEF*   tile_ls;
    TILEDEF*   tile_le;
    SPRITEDEF* sprite_ls;
    SPRITEDEF* sprite_le;
    Obviously, GFX_ENTRY, TILEDEF and SPRITEDEF are linked list structs. Each of them has a different set of data, but common to each are the next and prev pointers, and a char* name.

    Is it possible to create a single function to allocate and link a new entry into any of the three lists? I suspect void pointers may come into play here, but I'm not sure in what way.

    Cheers.

  2. #2
    and the hat of wrongness Salem's Avatar
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    But are the common elements in the same place in each structure?

    That is, could you create a union containing all the structures, with an additional union member overlaying the common (ie, the link pointers) members you want access to?
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
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  3. #3
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    I'd completely forgotten about unions. Unless I'm missing something, which is possible, it wouldn't be a problem...

    Here's how the structs presently look:
    Code:
    typedef struct sGFX_ENTRY
    {
      int n;
      char* name;
      char* filename;
      struct sGFX_ENTRY* next;
      struct sGFX_ENTRY* prev;
    } GFX_ENTRY;
    
    enum {sU,sD,sL,sR};
    
    typedef struct sTILEDEF
    {
      char *gfxname;
      char *name;
      char *message;           // message to display when moving at tile
      int prob;                 // probability of tile appearing after conclusive scan
      short int pain;          // degree of damage caused: 0-5
      short int spawntile;     // tile number to spawn
      short int spawndir[4];   // size of tilespawn area: up, down, left, right
      short int block;         // 0 - allow passage, non-0 - block passage
      short int cost;          // base price to remove this tile
      short int wear;          // base wear on digging instruments (spade, pickaxe)
      struct sTILEDEF* next;
      struct sTILEDEF* prev;
    } TILEDEF;
    
    typedef struct sSPRITEDEF
    {
      int nframes;
      char **gfxname;
      char *name;
      struct sSPRITEDEF* next;
      struct sSPRITEDEF* prev;
    } SPRITEDEF;
    I assume I can can drop next, prev and name from each of the structs and do something inelegant like this:
    Code:
    typedef struct sLIST
    {
      union
      {
        GFX_ENTRY* gfx_entry;
        TILEDEF* tiledef;
        SPRITEDEF* spritedef;
      }
      char *name;
      struct sLIST* next;
      struct sLIST* prev;
    } LIST;
    I'm not sure if that's what you were suggesting. It appears to make things even more complicated, unless there's a simpler way of managing that union than I'm aware.

    As a backup plan, I can just merge all the data into one generic struct and set the new entry function to take a parameter determining whether it's a GFX, SPRITE or TILE and leave unnecessary variables blank.

  4. #4
    and the hat of wrongness Salem's Avatar
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    Well if you're able to delete members from the structs you have, I guess you can also rearrange them as well (to put name, next, prev at the start)

    So
    Code:
    typedef union uNODE {
        GFX_ENTRY    gfx_entry;
        TILEDEF      tiledef;
        SPRITEDEF    spritedef;
        struct {
            union uNODE *next;
            union uNODE *prev;
            char *name;
        } any;
    } NODE;
    It's probably not so useful unless you want to do lots of casting.



    Another way would be to do
    Code:
    struct node {
        void *prev;
        void *next;
        char *name;
    };
    
    typedef struct sGFX_ENTRY
    {
      struct node link;
      int n;
      char* filename;
    } GFX_ENTRY;
    To use your linked list functions, you would just use &gfx_var.link as the parameter to your list functions.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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