How do I fix an Invalid Comparator error?

Question

MSVS 16.9.3

Win7-64

I get an invalid comparator error on the second execution of my sort comparator passed to the C++ Sort function provided in <algorithm> . I don't understand why I am getting this error: The sort routine call is:

sort(sorted.begin(), sorted.end(), remsort);

sorted is defined like this:

vector<ADI::RDA_Asset*>& sorted = *(new vector<ADI::RDA_Asset*>);

These are the three versions of remsort that I used:

Version 1: always works:

bool HOAReports::remsort(ADI::RDA_Asset* lhs, ADI::RDA_Asset* rhs) { 
   return (lhs->getRem() < rhs->getRem());
};

Version 2: works on the first call to remsort by the sort routine, fails on the second:

bool HOAReports::remsort(ADI::RDA_Asset* lhs, ADI::RDA_Asset* rhs) { 
   return (  (lhs->getRem() < rhs->getRem())
          || ((lhs->getCatName()).compare(rhs->getCatName()) < 0)
          || ((lhs->getRDAName()).compare(rhs->getRDAName()) < 0)
          };

Version 3: works on the first call to remsort by the sort routine, fails on the second:

bool HOAReports::remsort(ADI::RDA_Asset* lhs, ADI::RDA_Asset* rhs) {
   bool return_value = (  (lhs->getRem() < rhs->getRem())
                      || ((lhs->getCatName()).compare(rhs->getCatName()) < 0)
                      || ((lhs->getRDAName()).compare(rhs->getRDAName()) < 0)
                       ); 
   return return_value;
};

Version 2/3 have the same functionality. On the first and second call to remsort only ( (lhs->getRem() < rhs->getRem() ) is executed and the return_value is true. Looking at the failed assertion, it looks like the assertion is checked on both calls but fails on the second.

The MSVS code which fails is:

// FUNCTION TEMPLATE _Debug_lt_pred
template <class _Pr, class _Ty1, class _Ty2,
    enable_if_t<is_same_v<_Remove_cvref_t<_Ty1>, _Remove_cvref_t<_Ty2>>, int> = 0>
constexpr bool _Debug_lt_pred(_Pr&& _Pred, _Ty1&& _Left, _Ty2&& _Right) noexcept(
    noexcept(_Pred(_Left, _Right)) && noexcept(_Pred(_Right, _Left))) {
    // test if _Pred(_Left, _Right) and _Pred is strict weak ordering, when the arguments are the cv-same-type
    const auto _Result = static_cast<bool>(_Pred(_Left, _Right));
    if (_Result) {
        _STL_VERIFY(!_Pred(_Right, _Left), "invalid comparator");
    }

    return _Result;
}

Answer 1

Your comparison function does not fulfill the strict weak ordering requirements. See C++ named requirements: Compare .

In your specific case, you could implement it like this:

bool HOAReports::remsort(ADI::RDA_Asset* lhs, ADI::RDA_Asset* rhs) {
    if(lhs->getRem() < rhs->getRem()) return true;
    if(rhs->getRem() < lhs->getRem()) return false;

    // if we get here, lhs.getRem() == rhs.getRem()

    auto cmp = lhs->getCatName().compare(rhs->getCatName());
    if(cmp) return cmp < 0;

    // if we get here, lhs->getCatName() == rhs->getCatName()

    return lhs->getRDAName() < rhs->getRDAName();
}

Demo

Or simpler, use std::tuple or std::tie :

#include <tuple>

bool HOAReports::remsort(ADI::RDA_Asset* lhs, ADI::RDA_Asset* rhs) {
    return
        std::tuple{lhs->getRem(), lhs->getCatName(), lhs->getRDAName()}
        <
        std::tuple{rhs->getRem(), rhs->getCatName(), rhs->getRDAName()};
}