Issue with double pointer in LINKED LIST

Question

I'm trying to make a data structure which contains all the monsters in the game. For some reason when I allocate new monster i get "cannot convert Monster** to p_monster {Aka monster*} in initialization. If you could help me out it would be much appreciated. Thanks in advance

struct Monster {
    int x;
    int y;
    int health;
    Monster *next;
};
typedef Monster* p_monster;
class gameUtils {
protected:
  p_monster monster};
public:
gameUtils(){
monster=NULL;
...}
function(){
monster = getMonster(monster)}  // so that I can assign whatever value I want to monster->last->next through the function itself

p_monster getMonster(p_monster monster){
  p_monster newMonster = new p_monster;
        if(monster==NULL){
            monster=newMonster;

        }else{
    ....   // find last monster then create new one and pass it to caller

        return monster;   // returns new monster without any value
    }

Give yourself a break and use std::list<Monster>. The std::list has already been tested and works, so that you can move on and do other coding. — Thomas Matthews
– Thomas Matthews, Commented Jul 28, 2022 at 17:24
p_monster newMonster = new p_monster; should be p_monster newMonster = new Monster; I think you confused yourself with the unnecessary typedef — drescherjm
– drescherjm, Commented Jul 28, 2022 at 17:26
Fyi, p_monster {Aka monster*} - that you are compelled to mention that is an indication of what a terrible idea hiding pointer types in typedef aliases really is. And if you don't think so, look at @drescherjm ' s comment above. I suspect he's hyper-accurate in that assessment. — WhozCraig
– WhozCraig, Commented Jul 28, 2022 at 17:26
Type-aliasing pointers is one of those things that everyone discovers to be a great idea – for two or three weeks or so. And then the pain sets in... — molbdnilo
– molbdnilo, Commented Jul 28, 2022 at 17:33

463035818_is_not_an_ai · Accepted Answer · 2022-07-28 17:59:21Z

Type aliases are slightly more clear with using.

//typedef Monster* p_monster;
using p_monster = Monster*;

p_monster is Monster*. Now when you call new T then an object of type T is created and as a result you get a pointer to that object, a T*.

When you write:

p_monster newMonster = new p_monster;

Then new dynamically allocates a p_monster, a Monster*, and you get a p_monster*, a Monster**. There is no conversion from Monster** to Monster*, hence the error.

I suggest a std::vector<Monster> to store your monsters. Your approach for the list is intrusive, a Monster in your design is aware of being in a linked list, it has a next member. When you use a std::vector<Monster> you can store Monsters in a vector, outside of the vector, or in a different container, the Monster needs not care. Also not using manual memory managment via raw pointers will make your code much simpler.

struct Monster {
    int x;
    int y;
    int health;
    Monster(int x,int y,int health) : x(x),y(y),health(health) {}
};

struct gameUtils {
    std::vector<Monster> monster;
    Monster& createMonster(int x,int y,int health) {
        monster.emplace_back(x,y,health);
        return monster.back();
    }
};

A linked list in C++ is std::list, though a vector profits a lot from its elements being stored in contiguous memory. It profits from that so much, that it is often superior to a std::list even though time complexity of some methods suggests otherwise.

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