How to use struct/std::pair correctly? - c++

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);
}
};

Answer 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;