How to find max element of a vector with user defined predicate

Question

I need to find the maximum element of a vector. I want to use max_element from the STL library.

The code I have tried is:

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

class A
{
public:
    A(int _size, long _len) : size(_size), len(_len){}

    int size;
    long len;
};

bool CompareMaxA(const A& _a, const A& _b)
{
    return _a.len > _b.len;
}

int main() {

    vector<A> vec;
    A a1(3, 10);
    A a2(5, 30);
    vec.push_back(a1);
    vec.push_back(a2);

    auto it = max_element(vec.begin(), vec.end(), CompareMaxA);

    cout << "max = " << it->len;

    return 0;
}

I get max = 10, instead of max = 30. Why?

Answer 1

Your comparison function does the wrong thing. It returns true whenever _a.len is greater than _b.len, but std::max_element requires a custom comparison function that returns true in exactly the opposite scenarios. From cppreference on std::max_element:

Parameters

[...]

comp - comparison function object (i.e. an object that satisfies the requirements of Compare) which returns ​true if the first argument is less than the second.

You can fix it by

bool lessThanByLen(const A& _a, const A& _b)
{
    return _a.len < _b.len;
}

auto it = max_element(vec.begin(), vec.end(), lessThanByLen);

Custom comparators used by/passed to the standard library for order relations always seek for a less-than relation. In this case, the algorithm name (max_element) indicates that a maximum shall be found.

Note that as @JeJo pointed out in the comments, you can also consider passing a lambda:

auto it = max_element(vec.begin(), vec.end(),
    [](const A& lhs, const A& rhs) { return lhs.len < rhs.len; });