How use std::multiset with multiple comparator function? - c++

Question

Good afternoon, I have a C++ class Range which implements a operator < for use by std::multiset<Range> ranges_type.
Since the multiset constructor don't specify a a custom comparator functor, it uses the std::less operator <.
However, I need to use a second comparator functor for std::multiset ranges_type. Specifically, I would specify a second comparator:
std::multiset<Range, PointerCompare> where struct PointerCompare looks this :
struct PointerCompare{
bool operator()(const Range& a, const Range& b) const {
return (a.mPtr == b.mPtr)
}
Is it possible to use std:multiset with multiple comparator functions or is there a workaround? Thank you
The class Range looks this:
class Range {
public:
explicit Range(int item){
mLow = item;
mHigh = item;
mPtr = 0;
}
Range(int low, int high, char* ptr = 0,char* mapptr = 0){
mLow = low;
mHigh = high;
mPtr = ptr;
}
Range(void){
mLow = 0;
mHigh = 0;
mPtr = 0;
}
Range(const Range& r):
mLow(r.mLow),
mHigh(r.mHigh),
mPtr(r.mPtr)
{
}
bool operator==(const Range& rhs) const{
return (mLow <= rhs.mLow && mHigh >= rhs.mHigh);
}
bool operator<(const Range& rhs) const{
return mHigh < rhs.mHigh;
}
int low() const { return mLow; }
int high() const { return mHigh; }
char* getPtr() const { return mPtr; }
private:
int mLow;
int mHigh;
char* mPtr;
}; // class Range

Answer 1

Sounds almost like you'd be better if you used something from Boost::MultiIndex rather than trying to force several different comparator functions onto a std::multiset. They have a bunch of different container types (see here.) In particular I'd look at the ordered_indices versions.

Answer 2

I may have found a workaround for multiple comparator functions: Here it is:
Range targetRange = Range(PreviousNCopy,PreviousN, TmpPrevMapPtr);
bool Found = std::binary_search( ranges_type.begin(), ranges_type.end(),
targetRange, MyComparator() );
where: MyComparator is a struct :
struct MyComparator {
bool operator () ( const Range& d1, const Range& d2 ) const
{
return d1.getPtr() < d2.getPtr();
}
};
std::binary_search take o(log n) time but the std::multiset ranges_type must always remain sorted. Thank you.