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I'm programming a 2D RPG game that uses an array of derived class instance pointers. For clarity I provide a code example in C++. I would like to sort the elements of the entities array based on the worldY value of each element.

main.cpp:
<pre lang="C++">
/*
 * result:
 *    player
 *    object
 *    object
 *    npc
 *    npc
 *    2 npc y: 30
 *    1 npc y: 6
 *    1 object y: 20
 *    2 object y: 10
 *    player y: 0
*/

#include <iostream>
#include <vector>
#include <algorithm>
#include "Entity.h"
#include "Npc.h"
#include "Player.h"
#include "Object.h"

int main()
{
    Player* player = new Player();

    std::vector<Entity*> objs;
    std::vector<Entity*> npcs;
    std::vector<Entity*> entities;

    Entity* tmp = new OBJ_o();
    objs.push_back(tmp);
    objs.at(0)->worldY = 20;
    objs.at(0)->ID = "1";

    tmp = new OBJ_o();
    objs.push_back(tmp);
    objs.at(1)->worldY = 10;
    objs.at(1)->ID = "2";

    tmp = new NPC();
    npcs.push_back(tmp);
    npcs.at(0)->worldY = 6;
    npcs.at(0)->ID = "1";

    tmp = new NPC();
    npcs.push_back(tmp);
    npcs.at(1)->worldY = 30;
    npcs.at(1)->ID = "2";

    //--------------- update -------------------
    player->update();

    for (auto iter_objs = objs.begin(); iter_objs != objs.end(); ++iter_objs)
    {
        (*iter_objs)->update();
    }

    for (auto iter_npcs = npcs.begin(); iter_npcs != npcs.end(); ++iter_npcs)
    {
        (*iter_npcs)->update();
    }

    //-------------- entities --------------------
    entities.push_back(player);

    for (int i = 0; i < objs.size(); i++)
    {
        if (!objs.at(i)->name.empty())
        {
            entities.push_back(objs.at(i));
        }
    }

    for (int i = 0; i < npcs.size(); i++)
    {
        if (!npcs.at(i)->name.empty())
        {
            entities.push_back(npcs.at(i));
        }
    }

    // sort entities
    std::sort(entities.begin(), entities.end());

    //---------------- draw entities ------------------
    for (auto iter = entities.begin(); iter != entities.end(); ++iter)
    {
        std::cout << (*iter)->ID << " ";
        (*iter)->draw();
    }

    return 0;
}

#pragma once
#include <iostream>

class Entity
{
public:
    int worldY = 0;
    std::string name;
    std::string ID;

public:
    virtual void update() { std::cout << "base" << std::endl; }
    virtual void draw() { std::cout << "base y: -\n"; }

    // Overloading < operator for std::sort in main: draw
    bool operator<(const Entity& obj) const
    {
        return worldY < obj.worldY;
    }
};

#pragma once
#include "Entity.h"

class NPC : public Entity
{
public:
    NPC();
    void update() override;
    void draw() override;
};

#include "Npc.h"
#include <iostream>

NPC::NPC() { name = "npc"; }
void NPC::update() { std::cout << name << std::endl; }
void NPC::draw() { std::cout << "npc y: " << worldY << std::endl; }

#pragma once
#include "Entity.h"

class Player : public Entity
{
public:
    Player();
    void update() override;
    void draw() override;
};

#include "Player.h"
#include <iostream>

Player::Player() { name = "player"; }
void Player::update() { std::cout << name << std::endl; }
void Player::draw() { std::cout << "player y: " << worldY << std::endl; }

#pragma once

#include "Entity.h"

class OBJ_o : public Entity
{
public:
    OBJ_o();
    void update() override;
    void draw() override;
};

#include "Object.h"
#include <iostream>

OBJ_o::OBJ_o() { name = "object"; }
void OBJ_o::update() { std::cout << name << std::endl; }
void OBJ_o::draw() { std::cout << "object y: " << worldY << std::endl; }

Using:
std::sort(entities.begin(), entities.end());

I don't get the desired result:

player
object
object
npc
npc
player y: 0
1 npc y: 6
2 object y: 10
1 object y: 20
2 npc y: 30