How can I search an std::map using a key of a different type

Question

If I have std::map<X, Blah>, what is the best way of looking up a matching item in the map using an instance of Y?

Assume the information in Y is enough to uniquely find an X, but for performance reasons I don't want to create an instance of X by copying Y values.

I realize I can do this by creating a common base class or interface for X and Y and making that the map key, but is there any other way? e.g. creating some sort of comparator object?

Here is the sample code for clarity:

class X
{
public:
    int id;
    int subId;
};

std::map<X, Details> detailsMap;

class Y
{
public:
    int getId();
    int getSubId();
    int someOtherUnrelatedThings1;
    int someOtherUnrelatedThings2;
};

Now, if I have an instance of Y, in principle I should be able to find matching items in my map, given I can get an id and subId pair. But can I do it without creating an instance of X and copying over the id and subId?

Answer 1

With C++14 you can use heterogeneous lookup.

If you'd like to find an element with key that compares equivalent to the argument of std::map::find, you should provide a Comparator as a third template parameter which should have Comparator::is_transparent denoted as a type. It should also contain bool operator() comparing your map key with any other type you'd like.

Funny description aside, here's an example:

struct X
{
    int id;
    int subid;
};

struct Details {};

struct Comparator
{
    using is_transparent = std::true_type;

    // standard comparison (between two instances of X)
    bool operator()(const X& lhs, const X& rhs) const { return lhs.id < rhs.id; }

    // comparison via id (compares X with integer)
    bool operator()(const X& lhs, int rhs) const { return lhs.id < rhs; }
    bool operator()(int lhs, const X& rhs) const { return lhs < rhs.id; }

    // Same thing with Y
    bool operator()(const X& lhs, const Y& rhs) const { return lhs.id < rhs.getId(); }
    bool operator()(const Y& lhs, const X& rhs) const { return lhs.getId() < rhs.id; }
};

int main()
{
    std::map<X, Details, Comparator> detailsMap = {
        { X{1, 2}, Details{} }, 
        { X{3, 4}, Details{} },
        { X{5, 6}, Details{} }
    };

    // it1 and it2 point to the same element.
    auto it1 = detailsMap.find(X{1, 2});
    auto it2 = detailsMap.find(1);

    std::cout << detailsMap.size() << std::endl;
    std::cout << std::boolalpha << (it1 == detailsMap.end()) << std::endl; // false
    std::cout << std::boolalpha << (it1 == it2) << std::endl; // true
}

Note however that GCC didin't implement it until revision 219888.