Vectors, structs and std::find

Question

Again me with vectors. I hope I'm not too annoying. I have a struct like this :

struct monster 
{
    DWORD id;
    int x;
    int y;
    int distance;
    int HP;
};

So I created a vector :

std::vector<monster> monsters;

But now I don't know how to search through the vector. I want to find an ID of the monster inside the vector.

DWORD monster = 0xFFFAAA;
it = std::find(bot.monsters.begin(), bot.monsters.end(), currentMonster);

But obviously it doesn't work. I want to iterate only through the .id element of the struct, and I don't know how to do that. Help is greatly appreciated. Thanks !

Answer 1

You need to write your own search predicate:

struct find_monster
{
    DWORD id;
    find_monster(DWORD id) ; id(id) {}
    bool operator () ( const monster& m ) const
    {
        return m.id == id;
    }
};

it = std::find( monsters.begin(), monsters.end(), find_monster(monsterID));

Answer 2

find_if:

it = std::find_if(bot.monsters.begin(), bot.monsters.end(), 
        boost::bind(&monster::id, _1) == currentMonster);

Or write your own function object if you don't have boost. Would look like this

struct find_id : std::unary_function<monster, bool> {
    DWORD id;
    find_id(DWORD id):id(id) { }
    bool operator()(monster const& m) const {
        return m.id == id;
    }
};

it = std::find_if(bot.monsters.begin(), bot.monsters.end(), 
         find_id(currentMonster));

Answer 3

Take a look at the std::find template, the third parameter especially:

template<class InputIterator, class EqualityComparable>
InputIterator find(InputIterator first, InputIterator last,
               const EqualityComparable& value);

What is this EqualityComparable? Again from the documentation:

A type is EqualityComparable if objects of that type can be 
compared for equality using operator==, and if operator== is 
an equivalence relation.

Now, your type monster needs to define such an operator. If you don't the compiler generates one for you (as also the default ctor and the dtor) which does a memcmp sort of thing which doesn't work in your case. So, to use std::find first define a comparator function/functor that the algorithm can use to match your currentMonster i.e. something along the lines of:

 struct monster {
  // members
  bool operator==(const monster& l, const monster& r) const
  {
     return l.id == r.id;
  }
 };

Answer 4

how about:

std::find_if(monsters.begin(), 
             monsters.end(), 
             [&cm = currentMonster]
             (const monster& m) -> bool { return cm == m; }); 

Answer 5

or put the monsters in a map instead of a vector

or if they must be in a vector create an index map ie map of ID to vector index