I have tried numerous times how to make a do-while loop using the float constructor for my code but it seems it does not work properly as I wanted. For summary, I am making a TBRPG in C++ and I encountered few problems. But before that, let me post my code.

#include <iostream>
#include <string>
#include <ctime>
#include <cstdlib>

using namespace std;
    int char_level = 1; //the starting level of the character.
    string town; //town
    string town_name; //the name of the town the character is in.
    string charname; //holds the character's name upon the start of the game
    int gems = 0; //holds the value of the games the character has.
    const int MAX_ITEMS = 15; //max items the character can carry
    string inventory [MAX_ITEMS]; //the inventory of the character in game
    int itemnum = 0; //number of items that the character has.
    bool GameOver = false; //boolean intended for the game over scr.

    string monsterTroop [] = {"Slime", "Zombie", "Imp", "Sahaguin, Hounds, Vampire"}; //monster name
    float monsterTroopHealth [] = {5.0f, 10.0f, 15.0f, 20.0f, 25.0f}; // the health of the monsters
    int monLifeBox; //life carrier of the game's enemy troops
    int enemNumber; //enemy number


    //inventory[itemnum++] = "Sword";



    class RPG_Game_Enemy {
        public:
            void enemyAppear ()
            {
                srand(time(0));
                enemNumber = 1+(rand()%3); 
                    if (enemNumber == 1)
                        cout << monsterTroop[1]; //monster troop 1
                    else if (enemNumber == 2) 
                        cout << monsterTroop[2]; //monster troop 2
                    else if (enemNumber == 3)
                        cout << monsterTroop[3]; //monster troop 3
                    else if (enemNumber == 4)
                        cout << monsterTroop[4]; //monster troop 4

            }

            void enemDefeat () {
                cout << "The foe has been defeated. You are victorious." << endl;
            }

                    void enemyDies()
                { //if the enemy dies:
                //collapse declaration
                    cout << "The foe vanished and you are victorious!" << endl;
                }
};

         class RPG_Scene_Battle {
    public:
  RPG_Scene_Battle(float ini_health) : health (ini_health){};
  float getHealth()
    {
      return health;

      }
  void setHealth(float rpg_val){ health = rpg_val;};

private:
  float health;
};

//---------------------------------------------------------------//
// Conduct Damage for the Scene Battle's Damage
//---------------------------------------------------------------//

  float conductDamage(RPG_Scene_Battle rpg_tr, float damage) {
  rpg_tr.setHealth(rpg_tr.getHealth() - damage);
  return rpg_tr.getHealth();
  };

  // ------------------------------------------------------------- //

  void RPG_Scene_DisplayItem ()
  {
    cout << "Items: \n";
    for (int i=0; i < itemnum; ++i)
        cout << inventory[i] <<endl;
  };

In this code I have so far, the problem I have is the battle scene. For example, the player battles a Ghost with 10 HP, when I use a do while loop to subtract the HP of the character and the enemy, it only deducts once in the do while. Some people said I should use a struct, but I have no idea how to make it. Is there a way someone can display a code how to implement it on my game?

Edit:

I made the do-while by far like this:

do
  RPG_Scene_Battle (player, 20.0f);
  RPG_Scene_Battle (enemy, 10.0f);
  cout << "Battle starts!" <<endl;
  cout << "You used a blade skill and deducted 2 hit points to the enemy!"
  conductDamage (enemy, 2.0f);
while (enemy!=0)

also, I made something like this:

#include <iostream>
using namespace std;

int gems = 0;

class Entity {
public:
  Entity(float startingHealth) : health(startingHealth){}; // initialize health 
  float getHealth(){return health;}
  void setHealth(float value){ health = value;}; 
private:
  float health;
};


float subtractHealthFrom(Entity& ent, float damage) {
ent.setHealth(ent.getHealth() - damage);
return ent.getHealth();
};

int main () {

Entity character(10.0f);
Entity enemy(10.0f);

cout << "Hero Life: ";
cout << subtractHealthFrom(character, 2.0f) <<endl;
cout << "Monster Life: ";
cout << subtractHealthFrom(enemy, 2.0f) <<endl;
cout << "Hero Life: ";
cout << subtractHealthFrom(character, 2.0f) <<endl;
cout << "Monster Life: ";
cout << subtractHealthFrom(enemy, 2.0f) <<endl;

};

Struct method, they say, should solve this problem. How can I continously deduct hp from the enemy? Whenever I deduct something, it would return to its original value -_-