How to use struct/std::pair correctly?

Question

I am using struct in my project which has an unordered_map, I tried to put a struct or a pair of integers as key to the map, then manipulate the map, but couldn't compile, it says:

usr/include/c++/4.8/bits/hashtable_policy.h:1082:53: error: invalid use of incomplete type âstrruct std::hash<NAMESPACE::Span<int> >â
using __ebo_h1 = _Hashtable_ebo_helper<1, _H1>;

operator==(const unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>& __x,
 ^
/usr/include/c++/4.8/bits/unordered_map.h:1388:5: note:   template  argument deduction/substitution failed:
In file included from /usr/include/c++/4.8/string:48:0,

-----------------------------------------------------------
/usr/include/c++/4.8/bits/hashtable_policy.h: In instantiation of âstruct  std::__detail::_Hash_ccode_base<std::pair<int, int>, std::pair<const std::pair<int, int>, NAMESPACE::ConfigSet>, std::__detail::_Select1st, std::hash<std::pair<int, int> >, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, true>â:
/usr/include/c++/4.8/bits/unordered_map.h:100:18:   required from âclass std::unordered_map<std::pair<int, int>, RTBKIT::ConfigSet>â

What's wrong? Here is my struct:

namespace MYNAMESPACE {

template<typename T>
//can pass in a list than a range for IntervalFilter
struct Span
{
    //no arg/two args not working for the template typename T
    //span(T lb, T ub);
    //span();

    T lowerBound;
    T upperBound;

    static Span
    createFromJson(const Json::Value & val); 


    void fromJson(const Json::Value & val);

    Json::Value toJson() const;

    bool empty() const;
    //didn't declare this template type, isIncluded is not used
    template<typename U> bool isIncluded(const U & value) const;

 };
 } //namespace

 #endif

The code use the struct: ...

 private:
    static constexpr unsigned PRIORITY = 0x1401; }
    //std::unordered_map<Span<int>, ConfigSet> data;
    std::unordered_map<std::pair<int,int>, ConfigSet> data;
    void setConfig(unsigned configIndex, const AgentConfig& config) {
         auto range = [] (int first, int last) {
         return std::make_pair(first, last);
         };
         //int l = config.rangeFilter.lowerBound;
         //int r  = configrangeFilter.upperBound;
         //data[config.rangeFilter].set(configIndex);
         data[range(config.rangeFilter.lowerBound, config.rangeFilter.upperBound)].set(configIndex);
    }

};

You don't show any use of std::unordered_map in your code. — aschepler
– aschepler, Commented Apr 16, 2017 at 22:56

Peter Ruderman · Accepted Answer · 2017-04-16 23:57:51Z

1

The problem is that the standard does not define a hash specialization for std::pair. The compiler is telling you (indirectly) that it doesn't know how to generate hash values for your keys.

You should define your own hash class and use it in your data declaration. Something like this:

struct MyHash
{
  size_t operator()(std::pair<int, int> value) const
  {
    // I make no promise that this is a good hash function for your dataset.
    return value.first + value.second;
  }
};

std::unordered_map<std::pair<int, int>, ConfigSet, MyHash> data;

answered Apr 16, 2017 at 23:57

Peter Ruderman

12.6k1 gold badge41 silver badges59 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Jeffrey Cash Over a year ago

The only change I'd make is casting one of the values before the addition, as signed integer addition overflow is undefined behavior: return static_cast<ssize_t>(value.first) + value.second; Or you could do a better hash function like return (static_cast<size_t>(value.first) << sizeof(int)*8) | value.second;

Oneofkind Over a year ago

It compiled, thanks !, in order to find a pair in the map by key std::pair, do I need to use/declare bool operator== for the key(struct)? Or pair already able to compare if it's the same key?

Peter Ruderman Over a year ago

std::pair has built in comparison operators for lexicographic ordering. Its default operator== will work fine in your case.

Collectives™ on Stack Overflow

How to use struct/std::pair correctly?

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related