passing pointer to change its value but stay still

passing pointer to change its value but stay still

I am working on C++ with allegro library.

there is draw_tiles function.

void draw_tiles(def_small_world * s_world,BITMAP * dest){
    BITMAP *TileMap=NULL; 

    loadTilemap(s_world->tilemap,TileMap); 
    for(int y = 0;y<SIGHTY*2+1;y++)
    {
        for(int x = 0;x<SIGHTX*2+1;x++)
        {
            pasteTile(s_world->tile[y][x].kind,TileMap,dest,x,y);
        }
    }

}

and loadTilemap function.

void loadTilemap(int i,BITMAP * tileLayer){
    char c[128];
    sprintf(c,TILEPATHFORMAT,i);
开发者_运维问答    tileLayer= load_bitmap(c,NULL);
}

I expect

following code change TileMap to points somewhere

loadTilemap(s_world->tilemap,TileMap); 

but after loadTilemap, the TileMap variable stay still.

the following code works very well

char c[128];
sprintf(c,TILEPATHFORMAT,i);
tileLayer= load_bitmap(c,NULL);

tileLayer points 0x003f93f8

How to fix my code to TileMap points return value of load_bitmap?

---

You are passing the pointer by value, so a copy of the pointer is created. Inside loadTilemap, you are assigning a new value to the copy of the pointer - but that doesn't affect the original pointer.

Try passing the pointer by reference by changing the loadTilemap function signature to this:

void loadTilemap(int i,BITMAP *& tileLayer);

---

You need to pass a pointer to pointer to achieve that:

void loadTilemap(int i,BITMAP ** tileLayer){
   char c[128];
   sprintf(c,TILEPATHFORMAT,i);
   *tileLayer= load_bitmap(c,NULL); 
}

loadTilemap(s_world->tilemap, &TileMap); 

That's assuming TileMap is of type BITMAP *.

Alternatively, you could simply return the BITMAP* pointer as a result of loadTilemap:

 BITMAP* loadTilemap(int i,BITMAP * tileLayer){
   char c[128];
   sprintf(c,TILEPATHFORMAT,i);
   return load_bitmap(c,NULL); 
}

TileMap = loadTilemap(s_world->tilemap, TileMap);

This would allow you to get rid of tileLayer parameter altogether, as you don't seem to be using it for anything else in loadTileMap (i.e. it's only an output parameter).

---

Try this:

void loadTilemap(int i,BITMAP ** tileLayer){
    char c[128];
    sprintf(c,TILEPATHFORMAT,i);
    *tileLayer = load_bitmap(c,NULL);
}


loadTilemap(s_world->tilemap, &TileMap); 

The problem was that you pass the pointer to the BITMAP by value. To get the new pointer value out of loadTilemap, you have to pass it by reference.

EDIT:

On the other hand: why don't you just return the pointer to the newly created BITMAP?

BITMAP * loadTilemap(int i* tileLayer){
    char c[128];
    sprintf(c,TILEPATHFORMAT,i);
    return load_bitmap(c,NULL);
}

...

TileMap = loadTilemap(s_world->tilemap);