Comparator function error no match for call to function

Question

I have a function called FindMaxByArea that takes in a vector of Rectangle objects and returns one with the biggest length. When I compile my code I keep getting this error:

Rectangle.h:40:23: error: no match for call to ‘(AreaCompare) (const value_type&, const value_type&)’
         if (isLessThan(arr[maxIndex], arr[i]))
             ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~

I am also not sure if appending {} to the end of AreaCompare is the correct way to call this function. Any suggestions would be appreciated.

return findMax(arr, AreaCompare{});

This is my Rectangle.h file

#pragma once

#ifndef Rectangle_H
#define Rectangle_H

#include <vector>
#include <iostream>
#include <functional>


using namespace std;

/* 
    Rectangle class
*/
class Rectangle
{
public:
    int GetLength() const { return m_Length; }
    int GetWidth() const { return m_Width; }
    int getArea() const { return m_Length * m_Width; }
    int getPerimeter() const { return (m_Width * 2) + (m_Length * 2); }
    Rectangle(int length, int width) : m_Length(length), m_Width(width) {}

private:
    int m_Length;
    int m_Width;
};

/* 
    Function that takes in a vector and a compare function and
    returns a generic with the largest properties
*/
template <typename Object, typename Comparator>
const Object & findMax(const vector<Object> & arr, Comparator isLessThan)
{
    int maxIndex = 0;

    for (int i = 1; i < arr.size(); ++i)
        if (isLessThan(arr[maxIndex], arr[i]))
            maxIndex = i;

    return arr[maxIndex];
}

/*
    A compare function that compares the area of two rectangles
*/
class AreaCompare
{
public:
    bool isLessThan(const Rectangle &lhs, const Rectangle &rhs) const
    {
        return lhs.getArea() < rhs.getArea();
    }
};

/*
    A compare function that compares the perimeter of two rectangles
*/
class PerimeterCompare
{
public:
    bool isLessThan(const Rectangle &lhs, const Rectangle &rhs) const
    {
        return lhs.getPerimeter() < rhs.getPerimeter();
    }
};

/* 
    Function compares a vector of rectangles and returns one with
    the largest area
*/
template <typename Object>
const Object & FindMaxByArea(const vector<Object> & arr)
{
    return findMax(arr, AreaCompare{});
}

/* 
    Function compares a vector of rectangles and returns one with
    the largest perimeter
*/
template <typename Object>
const Object & FindMaxByPerim(const vector<Object> & arr)
{
    return findMax(arr, PerimeterCompare{});
}



#endif

main.cpp

#include <iostream>
#include <vector>
#include "Rectangle.h"

int main() {
vector<Rectangle> vec
    {
        Rectangle(1, 3),
        Rectangle(4, 4)
    };

cout << "Highest Area: " << FindMaxByArea(vec) << endl;

return 0;
}

How are you calling FindMaxByArea? (What type is the arr parameter you're passing to it?) — 1201ProgramAlarm
– 1201ProgramAlarm, Commented Apr 25, 2020 at 3:05
@1201ProgramAlarm I added main.cpp. arr is vector of Rectangles — asus
– asus, Commented Apr 25, 2020 at 3:14

1201ProgramAlarm · Accepted Answer · 2020-04-25 03:21:12Z

2

In findMax, Comparator isLessThan is a parameter. It is a variable like any other parameter. To call a member function of that type, you'll want to access it thru the variable:

if (isLessThan.isLessThan(arr[maxIndex], arr[i]))

And, since there are no class members, isLessThan could be a static member of the class.

As an alternative you can define a bool operator()(const Rectangle &lhs, const Rectangle &rhs) member function, then your original code would work.

answered Apr 25, 2020 at 3:21

1201ProgramAlarm

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