C++: “invalid comparator” assert [duplicate]

Question

Here is the code:

struct Payment
{
    Payment(time_t time, float money) : mTime(time), mMoney(money) {}
    bool operator==(const Payment& p) const // exact comparison
    {
        return mTime == p.mTime && mMoney == p.mMoney;
    }
    time_t  mTime;
    float   mMoney;
};

std::vector<Payment>    payments;

auto sortP = [](const Payment& p1, const Payment& p2) { return p1.mTime < p2.mTime || p1.mMoney <= p2.mMoney; };
std::sort(payments.begin(), payments.end(), sortP);

std::sort (not always, but sometimes, when mTime of two elements close to each other) raises invalid comparator assert in Visual Studio 2015. What's wrong with the code?

Answer 1

The problem is with the implementation of sortP. It does not satisfy the strictly weak ordering criteria. Read the details at https://www.boost.org/sgi/stl/StrictWeakOrdering.html.

I suggest the following change:

auto sortP = [](const Payment& p1, const Payment& p2)
{
   // Order by mTime if they are not equal.
   if ( p1.mTime != p2.mTime)
   {
     return p1.mTime < p2.mTime;
   }

   // Otherwise, order by pMoney
   return ( p1.mMoney < p2.mMoney); // Use < not <=
};

You can use std::tie to make the implementation simpler.

auto sortP = [](const Payment& p1, const Payment& p2)
{
   return std::tie(p1.mTime, p1.mMoney) < std::tie(p2.mTime, p2.mMoney);
};