Sort algorithm c++

Question

#include <iostream>
#include <array>
#include <algorithm>
using namespace std;
class Test
{
private:
    int value;
public:
    Test()
    {
 
    }
    Test(int _value)
    {
        value = _value;
    }
    bool operator<(Test&);
 
};
bool Test::operator<(Test& rValue) {
    return this->value < rValue.value;
}
int main()
{
    Test* arr = new Test[950];
    arr[0] = Test(5);
    arr[1] = Test(10);
    arr[2] = Test(7);
    arr[3] = Test(3);
    arr[4] = Test(10);
    sort(arr, arr + 5, [](Test& a, Test& b) { return a < b ? false : true; });
}

Sort algorithm works perfectly until there are objects with same grade value.

P.S I know other way to use sort and reverse.

I'm using visual studio 2019

ERROR: Debug Assertion Failed! Expression: invalid comparator

Aside, a default Test object will have an unitialized value field. — cigien
– cigien, Commented Jul 11, 2020 at 17:37
The problem is that the comparator function yoyr passing to sort, return true for equal elemtents — Eduardo Pascual Aseff
– Eduardo Pascual Aseff, Commented Jul 11, 2020 at 17:52
You cannot negate operator< and use it for sorting, as !(a < b) is equal to a >= b. — Slava
– Slava, Commented Jul 11, 2020 at 18:17

john · Accepted Answer · 2020-07-11 17:44:25Z

6

A sort comparator must return false for items that are equal, yours returns true.

Try this instead.

sort(arr, arr + 5, [](Test& a, Test& b) { return b < a; });

answered Jul 11, 2020 at 17:44

john

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3 Comments

Wilf Rosenbaum Over a year ago

@BerianidzeLuka Debug assertions are not executed in Release mode.

Berianidze Luka Over a year ago

You said that sort comparator must return false for equal items,but how this problem is fixed in release? operator stays < like it is

john Over a year ago

If you do not return false for equal items then your program has undefined behaviour. Working is one possible outcome of undefined behaviour, but obviously you cannot rely on that. The program is still incorrect even if the bug does not show itself.

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