C++ pointer parameter trouble - c++

Question

I am a beginning programmer writing a graphical game using SDL. The function that splits a tile-sheet into sections or "clips" and puts it into a array and the function that draws specific "clips" onto the screen are not working as intended.
void split_tilesheet(int width, int height, int space, Entity * ent){
std::cout << "Splitting Tileset...";
int t_width = (width / SPR_W);
int t_height = (height / SPR_H);
int numTiles = (t_width * t_height);
ent = new Entity [numTiles + 1];
if( ent == NULL){
err("!failed to alloc!");
}else{
std::cout << "allocated"<< std::endl;
}
int count = 0;
for(int i = 0; i < t_width; i++){
for(int j = 0; j < t_height; j++){
ent[count].bounds.x = i * SPR_W;
ent[count].bounds.y = j * SPR_H;
ent[count].bounds.w = SPR_W;
ent[count].bounds.h = SPR_H;
ent[count].id = ent[i].x + ( ent[i].y * t_width);
count++;
}
}
}
void draw_room(char tiledata[MAP_MAX_X][MAP_MAX_Y], Entity * ent){
SDL_Rect bounds;
for(int x = 0; x < MAP_MAX_X; x++){
for(int y = 0; y < MAP_MAX_Y; y++){
if(tiledata[x][y] == '0' || tiledata[x][y] == ' ' || tiledata[x][y] == '\n' ){
draw_img(x * SPR_W , y * SPR_H, tiles, bounds, ent[0].bounds);
}
if(tiledata[x][y] == '1'){
draw_img(x * SPR_W , y * SPR_H, tiles, bounds, ent[1].bounds);
}
}
}
}
class Entity
{
public:
SDL_Rect bounds;
SDL_Surface* sprite;
int id;
int x;
int y;
int w, h;
};
I was trying to use pointers to dynamically allocate the memory at runtime.
The program compiles, but segfaults. gdb says that the segfault is due to the draw_room() function, but I cannot figure out why. The pointer I was passing to the draw_room function was:
Entity * floor0_clips = NULL;
This didn't work either
Entity * floor0_clips;
Please help...

Answer 1

C++ uses pass-by-value (unless you specify pass-by-reference), which you didn't.
A variable in a function is a copy of the argument given. For example:
int func(int x)
{
x = 5;
}
int main()
{
int y = 6;
func(y);
// here, `y` is still `6`
}
Your case is fundamentally the same as this. You send floor0_clips to a function, the function updates a copy of it, leaving the original unchanged.
To use pass-by-reference instead, put the & symbol just before the variable name in the function's parameter list, i.e. in your case Entity * &ent . Do not change anything in the code which calls the function; it is the function's parameter list declaration that decides whether the value is passed by value or by reference.
NB. You appear to be allocating too many Entities anyway (why the + 1?).