No matching call error for unordered_map

Question

My line:

unordered_map < pair<long long,long long>, long long> myMap;

And the error:

error: no matching function for call to 'std::unordered_map<std::pair<long long int, long long int>, long long int>::unordered_map()'

Code that reproduces the error:

#include <math.h>
#include <stdio.h>
#include <vector>
#include <algorithm>
#include <unordered_map>
#include <utility>
using namespace std;

unordered_map < pair<long long,long long>, long long> myMap;

int main() {
    return 0;
}

Did you #include <unordered_map> and #include <utility>? — Cory Kramer
– Cory Kramer, Commented Aug 31, 2015 at 18:48
Please include a copy of a minimal working example of your code to help people better diagnose your problem — 7VoltCrayon
– 7VoltCrayon, Commented Aug 31, 2015 at 18:49
Please edit your question with an minimal reproducible example or SSCCE (Short, Self Contained, Correct Example) — NathanOliver
– NathanOliver, Commented Aug 31, 2015 at 18:50
With another compiler, the message is "The C++ Standard doesn't provide a hash for this type.". Meaning std::pair. — Bo Persson
– Bo Persson, Commented Aug 31, 2015 at 18:55

Barry · Accepted Answer · 2015-08-31 18:55:53Z

10

std::unordered_map requires a hash functor in order to do anything. By default, that hash functor is is std::hash<Key> - but the standard only provides specializations for the integral types and pointers. std::pair<long long, long long> is neither, so effectively the compiler is telling you that you cannot instantiate the unordered_map you want because its hash functor is ill-formed.

What you need to do is provide your own. For example, here is the worst possible hash functor:

struct AlwaysZero {
    size_t operator()(pair<long long, long long> const& ) const {
        return 0;
    }
};

unordered_map < pair<long long,long long>, long long, AlwaysZero> myMap;

That compiles. It will also perform terribly. You could check out boost::hash_combine to get an idea of how to correctly write a much better hash.

answered Aug 31, 2015 at 18:55

Barry

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9 Comments

Sean Hill Over a year ago

It's possible that I am asking an XY question. Is there a better way to do a lookup by two variables? I'm basically trying to create a map where I feed it (a,b) and it returns c, and as efficiently as possible to minimize overall runtime.

Sean Hill Over a year ago

I also find this strange because using pairs works with map but not unordered_map

Barry Over a year ago

@user64283 Having a pair<a,b> for the key sounds reasonable to me if that's what you're doing. Your problem is just that the standard library doesn't support that by default.

Barry Over a year ago

@user64283 That sounds terrible.

Barry Over a year ago

@user64283 Because it's a hack. You want to look up (a,b) so look up (a,b), don't look up str(a) + " " + str(b). And doing math with numbers is muuuuch faster than doing math with strings. And you don't need to deal with memory allocation.

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