unordered_set: invalid operands to binary expression ('const Play' and 'const Play')

Question

I'm getting this error when I try to insert an element into an unordered_set:

error: invalid operands to binary expression ('const Play' and 'const Play')
        {return __x == __y;}

Here's a screenshot of the whole error: https://www.dropbox.com/s/nxq5skjm5mvzav3/Screenshot%202013-11-21%2020.11.24.png

This is my hash function:

struct Hash {

        size_t operator() (Play &play) {

            unsigned long hash = 5381;
            int c;

            string s_str = play.get_defense_name() + play.get_offense_name() + play.get_description();
            const char * str = s_str.c_str();

            while ( (c = *str++) )
                hash = ((hash << 5) + hash) + c; /* hash * 33 + c */

            cout << hash << endl;
            return hash;

       }
    };

This is where I declare the unordered_list:

unordered_set<Play, Hash> Plays;

And this is my overloaded == for my Play class:

friend bool operator== (Play& p1, Play& p2)
    {
        return 
            (p1.get_defense_name() == p2.get_defense_name()) && 
            (p1.get_offense_name() == p2.get_offense_name()) &&
            (p1.get_description() == p2.get_description()); 
    }

Any idea what could be going on here?

Thanks.

Answer 1

the error is saying it try to compare const Play with const Play but you only provide operator == for Play

friend bool operator== (const Play& p1, const Play& p2)
    {
        return 
            (p1.get_defense_name() == p2.get_defense_name()) && 
            (p1.get_offense_name() == p2.get_offense_name()) &&
            (p1.get_description() == p2.get_description()); 
    }

Answer 2

It looks like unordered set is looking to compare constant Play objects, but your == offers to compare non-const ones. Since dropping const-ness is not allowed, the compiler produces an error. Adding const-ness should fix the problem:

friend bool operator== (const Play& p1, const Play& p2)

You should add const to your operator() as well:

size_t operator() (const Play &play)