Multimap with objects

[unknown418]

I am confused with how to implement the multimap if both key and values are different objects. I don't get how to define this comparison function, where it should be and what it should do. If the keys are key_obj, and values are value_obj then you have
multimap<key_obj, value_obj, _______> mmap;

I can't seem to google up any examples.

[Banfa]

You basically have 2 options the first is to put a operator< overload in your key_obj class, if you do this then you can declare you map as

multimap<key_obj, value_obj> mmap;

The second is to subclass from less<class T> specialising for your class, something like

struct key_obj_less : less<key_obj>
{
    bool operator()(const key_obj & x, const key_obj & y) const
    {
        // Code here to determin with object is less than the other
    }
};
 

then you can declare your multimap as

multimap<key_obj, value_obj, key_obj_less> mmap;

[weaknessforcats]

multimap<key_obj, value_obj, _______> mmap;

The multimap requires a binary predicate for the comparator function. This function is called to compare two keys so the map can place the key in the right place.

However, you don't need this comparator function if:
1) the key_obj class can implement an operator<().
2) there is an operator<() that has two key_obj arguments.

Otherwise you can write the comparator yourself.

The comparator functon for multimap must be a binary predicate. That is any function you choose provided it returns a bool and has two key_obj arguments:

bool unknown418(key_obj first, key_obj second);
 

Then you create your multimap by supplying the address of this function:

multimap<key_obj, value_obj, unknown418> mmap;
 

[unknown418]

Thanks for the replys.
I tried writing my own compartor function but it didn't work, it said it wasnt' within scope.

What I want to do is actually have the keys and values be pointers to objects, so that I don't keep creating them. I might end up with > 1000 of these objects.
But even if I use the actual objects for the keys it still doesn't work.
This is what I have currently:

class key{
int x;
int y;
....
bool operator == (key& k){
   if((k.x == x &&  k.y==y)|(k.x == y && k.y==x))
      return true;
   else
      return false;
 
bool compare (key& k){
   if((k.x<x &&  k.y<y)|(k.x <y && k.y<x))
      return true;
   else
      return false;
 
bool compare(key* k){
   if((k->x < x &&  k->y<y)|(k->x < y && k->y<x))
      return true;
   else
      return false;
}
 
 
};
 
class value{
....
};
 
class top{
multimap<key*, value*, compare> mmap1;
multimap<key, value*, compare> mmap2;
}
 

I have also tried moving the compare functions inside the top class but it doesn't help.


When I create the iterator do use include the compare part or not?
In addition, if I want to use the find() function does the multimap calls the == operator for the object. Do I just overwrite this function.

[weaknessforcats]

Did you not read my Post #3????

Your compare function must be a binary predicate.

The compare function I see are not binary predicates.

Also, the compare nedd only be less than. The function is to return true of the first argument is less than the second argument.

Please read my post again.