Array of classes 'undefined' C++

So, I have a array of monsters and a function to display their names. The problem is the debugger says Mons(the array) is undefined. I'd like to know what I'm doing wrong. Please help! The errors are mostly Mons is undefined.

#include <iostream> 
#include <fstream>
#include <windows.h>
#include <time.h>

/**
RPG v0.1.6
-
**/

//-- WinAPI stuff
void ClearScreen()
  {
  HANDLE                     hStdOut;
  CONSOLE_SCREEN_BUFFER_INFO csbi;
  DWORD                      count;
  DWORD                      cellCount;
  COORD                      homeCoords = { 0, 0 };

  hStdOut = GetStdHandle( STD_OUTPUT_HANDLE );
  if (hStdOut == INVALID_HANDLE_VALUE) return;

  /* Get the number of cells in the current buffer */
  if (!GetConsoleScreenBufferInfo( hStdOut, &csbi )) return;
  cellCount = csbi.dwSize.X *csbi.dwSize.Y;

  /* Fill the entire buffer with spaces */
  if (!FillConsoleOutputCharacter(
    hStdOut,
    (TCHAR) ' ',
    cellCount,
    homeCoords,
    &count
    )) return;

  /* Fill the entire buffer with the current colors and attributes */
  if (!FillConsoleOutputAttribute(
    hStdOut,
    csbi.wAttributes,
    cellCount,
    homeCoords,
    &count
    )) return;

  /* Move the cursor home */
  SetConsoleCursorPosition( hStdOut, homeCoords );
  }



class Player {
    public:
    int HP;
    int MHP;
    int STR;
    int AGI;
    int RES;
    int XP;
    int LVL;
    Player() { HP = 8; MHP = 8; STR = 4; AGI = 3; RES = 2; XP = 0; LVL = 1; };
private:
};

struct DT {

void save_to_file(std::string filename, const Player& P)
{
    std::ofstream f( filename.c_str() );
    f << P.HP << std::endl;
    f << P.MHP << std::endl;
    f << P.STR << std::endl;
    f << P.AGI << std::endl;
    f << P.RES << std::endl;
    f << P.XP << std::endl;
    f << P.LVL << std::endl;
}

bool load_from_file(std::string filename, Player& P) {
  std::ifstream f( filename.c_str() );
  f >> P.HP;
  f >> P.MHP;
  f >> P.STR;
  f >> P.AGI;
  f >> P.RES;
  f >> P.XP;
  f >> P.LVL;
  if(f.good()){
    std::cout << "Success!" << std::endl;
  }
  else {
    std::cout << "Failure" << std::endl;
  }
  return f.good();
}

};

class Monster {
public:
    int HP;
    int MHP;
    int STR;
    int AGI;
    int RES;
    int XP;
    std::string name;
    ///For Calling
    Monster() { };
    ///Set variables for new battle
    Monster(int A, int B, int C, int D, int E, int F, std::string G){
    this->HP = A;
    this->MHP = B;
    this->STR = C;
    this->AGI = D;
    this->RES = E;
    this->XP = F;
    this->name = G;
    };
    void setStats(int A, int B, int C, int D, int E, int F, std::string G){
    this->HP = A;
    this->MHP = B;
    this->STR = C;
    this->AGI = D;
    this->RES = E;
    this->XP = F;
    this->name = G;
    };
};

int monstersalive = 3;




// -- Call Functions
void BtlDisplay(Player& P, Mons M[]);
void Battle(Player& P, Mons M[]);

int main()
{
    // -- Call Classes
    Player p;
    DT d;

    Monster Mons[3];
    for(int j = 0; j < 3; j++){
        Mons[j].setStats(9,9,9,9 + j,9,9,"Noob");
    }

    BtlDisplay(p,Mons);


    //Battle(p,m);


    d.save_to_file("GameData1.txt", p);
    d.save_to_file("GameData2.txt", p);
    d.save_to_file("GameData3.txt", p);



    return 0;
}

void BtlDisplay(Player& P, Mons M[]) //-- Displays battle stats
{
    ClearScreen();
    std::cout << "---------" << std::endl;
    std::cout << "L " << P.LVL << std::endl;
    std::cout << "HP " << P.HP << "/" << P.MHP << std::endl;
    std::cout << "---------\n\n" << std::endl;
    std::cout << "Monsters\n--------" << std::endl;
    for(int i = 0; i < 3; i++){
        if(M[i].HP > 0){

        }
    }
}

void Battle(Player& P, Mons M[])
{
    while(monstersalive > 0 && P.HP > 0){
        srand(time(NULL));
    }
}