使用 .find() 和 struct 作为 C++ 映射中的键

Question

I do not have access to BOOST or STL; my struct and map looks similar to the following psuedo:

 struct s_map_key{
    int a;
    int b;
    bool operator<(const s_map_key& smk) const 
    {
        if (a < smk.a)       
        {            
            return true;
        } else if (a == smk.a)  
        { 
            if (b < smk.b) 
            { 
                return true;
            } 
            else if (b == smk.b)
            {
                return true;
            }
        } 
            return false;
    }
};

int main(int argc, char* argv[])
{

    std::multimap<s_map_key, std::string> myMap;
    for(int i = 0; i <10; i++)
    {
    s_map_key smk;
    smk.a = i;
    smk.b = 2;
    myMap.insert(std::make_pair(smk, "test"));
    }

    s_map_key smk;
    smk.a = 3;
    std::multimap<s_map_key, std::string>::iterator x = myMap.find(smk);
    if(x != myMap.end())
    {
        std::cout << x->first.a <<std::endl;
        std::cout << x->first.b <<std::endl;
    }
    return 0;
}

What I am trying to do is search my multimap for all cases where A = 2, B = 2, or A & B = 2. I am not exactly sure but, I think i need to create the predicates in my struct for "finding". Ideas?

Answer 1

operator< is all you need for find or anything else. However, your implementation has a bug. It returns true if the operands are equal.

find and other Standard Library components use the assumption that a == b iff ! (a < b) && ! (b < a) a == b iff ! (a < b) && ! (b < a) a == b iff ! (a < b) && ! (b < a) , so if a and b are equal, < must be false for find to work.

        else if (b == smk.b)
        {
            return false; // was true
        }

Answer 2

Unless you want to establish an equivalence of 2 == smk if (smk.a == 2 || smk.b == 2 || smk.a & smk.b == 2), I suggest making the predicate external and using std::find_if. If you make it part of the struct so that you can use multimap's internal find function, you could open the door for the predicate being used in an undesired way by another coder. Also, the internal find function will only return the first instance that matches, whereas with std::find_if you can update your starting point so that you can find all instances. If you want to use the internal functions, you'll need to use equal_range instead.