将 std::sort 与自定义 class 一起使用

Question

I have created a minimum reproducible example below:

Polygon3D.h:

#pragma once
class Polygon3D
{
public:
    // Constructors
    Polygon3D();
    Polygon3D(const Polygon3D& p);
    // Accessors
    void SetAverageZ(float average);
    float GetAverageZ() const;
    // Operators
    Polygon3D& operator= (const Polygon3D& rhs);
    const bool operator< (const Polygon3D& rhs) const;
private:
    float _averageZ;
};

Polygon3D.cpp file:

#include "Polygon3D.h"
// Default constructor
Polygon3D::Polygon3D()
{
    _averageZ = 0;
}
// Copy constructor
Polygon3D::Polygon3D(const Polygon3D& p)
{
    _averageZ = p.GetAverageZ();
}
// Accessors
void Polygon3D::SetAverageZ(float average)
{
    _averageZ = average;
}
float Polygon3D::GetAverageZ() const
{
    return _averageZ;
}
// Operators
Polygon3D& Polygon3D::operator=(const Polygon3D& rhs)
{
    return *this;
}
// Used by std::sort to order lists of this class
const bool Polygon3D::operator<(const Polygon3D& rhs) const
{
    return _averageZ < rhs.GetAverageZ();
}

Sort.cpp file:

#include <iostream>
#include <vector>
#include <algorithm>
#include "Polygon3D.h"

int main()
{
    std::vector<Polygon3D> list;

    Polygon3D temp1 = Polygon3D();
    temp1.SetAverageZ(2);
    list.push_back(temp1);

    Polygon3D temp2 = Polygon3D();
    temp2.SetAverageZ(0);
    list.push_back(temp2);

    Polygon3D temp3 = Polygon3D();
    temp3.SetAverageZ(6);
    list.push_back(temp3);

    Polygon3D temp4 = Polygon3D();
    temp4.SetAverageZ(5);
    list.push_back(temp4);

    std::sort(list.begin(), list.end(), std::less<Polygon3D>());

    std::cout << "Sorted vector: ";

    for (Polygon3D poly : list) 
    {
        std::cout << poly.GetAverageZ();
    }
}

And this is the output I receive:

Sorted vector: 2065

As you can see, the vector is not being sorted and is output in the order the items are added. Is there a reason why this is not being sorted correctly? Any help is greatly appreciated.

Answer 1

You have defined a copy assignment operator which doesn't actually copy assign the _averageZ member. std::sort will be using that to exchange elements of the vector, but because it doesn't actually change any members, the sorting of the _averageZ members in the vector will be unchanged.

You need to always implement the special member functions properly when you declare them.

The compiler does however define a copy assignment operator that does the correct thing for you implicitly as long as certain conditions are met .

Just remove your user-provided copy assignment overload and copy constructor definitions and declarations from the class and it will work as expected, ie remove Polygon3D(const Polygon3D& p); and Polygon3D& operator= (const Polygon3D& rhs); and their implementations. Then the compiler will define a correctly working copy constructor and copy-assignment constructor for you implicitly.

But if you must have these special member functions declared, then you need to actually copy the _averageZ member in the copy assignment overload.