Question

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);
    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?

Answer 1

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);