forward class declaration

Can anybody help me with the following error on build. I thought if you only use a reference to a class type you can place a forward decleration.
player.cpp(111): error C2027: use of undefined type 'Monster'

EDIT: The problem is solved by #including Monster.h in Player.cpp and #including Player.h in Monster.cpp. So do you need to include header files of classes in .cpp files regardless of wether they are passed by reference?

this is Player.h

//Player.h
#ifndef PLAYER_H
#define PLAYER_H

#include "Weapon.h"
#include <string>

class Monster;

class Player{
public:
	Player();
	bool isDead();
	std::string getName();
	int getArmor();
	void takeDamage(int damage);
	void createClass();
	bool attack(Monster& monster);
	void levelUp();
	void rest();
	void viewStats();
	void victory(int xp);
	void gameover();
	void displayHitPoints();
private:
	std::string mName;
	std::string mClassName;
	int mAccuracy;
	int mHitPoints;
	int mMaxHitPoints;
	int mExpPoints;
	int mNextLevelExp;
	int mLevel;
	int mArmor;
	Weapon mWeapon;
};

#endif

this is Player.cpp.

//Player.cpp
#include "Player.h"
#include "Random.h"
#include "Weapon.h"
#include <iostream>
using namespace std;

Player::Player(){
	mName = "Default";
	mClassName = "Default";
	mAccuracy = 0;
	mHitPoints = 0;
	mMaxHitPoints = 0;
	mExpPoints = 0;
	mNextLevelExp = 0;
	mLevel = 0;
	mArmor = 0;
	mWeapon.mName = "Default Weapon Name";
	mWeapon.mDamageRange.mHigh = 0;
	mWeapon.mDamageRange.mLow = 0;
}

bool Player::isDead(){
	return mHitPoints <= 0;
}

int Player::getArmor(){
	return mArmor;
}

void Player::takeDamage(int damage){
	mHitPoints -= damage;
}

void Player::createClass(){
	cout << "CHARACTER CLASS GENERATION" << endl;
	cout << "==========================" << endl;

	cout << "Enter your character's name: ";
	getline(cin,mName);

	cout << "Please select a character class number..." << endl;
	cout << "1)Fighter 2)Wizard 3)Cleric 4)Thief: ";
	int characterNum = 1;
	cin  >> characterNum;

	switch(characterNum){
	case 1:
		mClassName = "Fighter";
		mAccuracy = 10;
		mHitPoints = 20;
		mMaxHitPoints = 20;
		mExpPoints = 0;
		mNextLevelExp = 1000;
		mLevel = 1;
		mArmor = 4;
		mWeapon.mName = "Long Sword";
		mWeapon.mDamageRange.mLow = 1;
		mWeapon.mDamageRange.mHigh = 8;
		break;
	case 2:
		mClassName = "Wizard";
		mAccuracy = 5;
		mHitPoints = 10;
		mMaxHitPoints = 10;
		mExpPoints = 0;
		mNextLevelExp = 1000;
		mLevel = 1;
		mArmor = 1;
		mWeapon.mName = "Staff";
		mWeapon.mDamageRange.mLow = 1;
		mWeapon.mDamageRange.mHigh = 4;
		break;
	case 3:
		mClassName = "Cleric";
		mAccuracy = 8;
		mHitPoints = 15;
		mMaxHitPoints = 15;
		mExpPoints = 0;
		mNextLevelExp = 1000;
		mLevel = 1;
		mArmor = 3;
		mWeapon.mName = "Flail";
		mWeapon.mDamageRange.mLow = 1;
		mWeapon.mDamageRange.mHigh = 6;
		break;
	default:
		mClassName = "Thief";
		mAccuracy = 7;
		mHitPoints = 12;
		mMaxHitPoints = 12;
		mExpPoints = 0;
		mNextLevelExp = 1000;
		mLevel = 1;
		mArmor = 2;
		mWeapon.mName = "Short Sword";
		mWeapon.mDamageRange.mLow = 1;
		mWeapon.mDamageRange.mHigh = 6;
		break;
	}
}

bool Player::attack(Monster& monster){
	int selection = 1;
	cout << "1) Attack, 2) Run: ";
	cin  >> selection;
	cout << endl;

	switch(selection){
	case 1:
		cout << "You attack a(n) " << monster.getName() << " with a(n) "
			 << mWeapon.mName << endl;
		if(Random(0,20) < mAccuracy){
			int damage = Random(mWeapon.mDamageRange);
			int totalDamage = damage - monster.getArmor();
			if(totalDamage <= 0){
				cout << "Your attack failed to penetrate " << monster.getName()
					 << "'s armor." << endl;
			}
			else{
				cout << "You attack for " << totalDamage << " damage!" << endl;
				monster.takeDamage(totalDamage);
			}
		}
		else{
			cout << "You miss!" << endl;
		}
		cout << endl;
		break;
	case 2:
		if(Random(1,4) == 1){
			cout << "You run away!" << endl;
			return true;
		}
		else{
			cout << "You could not escape!" << endl;
			break;
		}
	}
	return false;
}

void Player::levelUp(){
	if(mExpPoints >= mNextLevelExp){
		cout << "You gained a level!" << endl;
		mLevel++;
		mNextLevelExp = mLevel * mLevel * 1000;
		mAccuracy += Random(1,3);
		mMaxHitPoints += Random(2,6);
		mArmor += Random(1,2);
		mHitPoints = mMaxHitPoints;
	}
}

void Player::rest(){
	cout << "Resting..." << endl;

	mHitPoints = mMaxHitPoints;
}

void Player::viewStats(){
	cout << "PLAYER STATS" << endl;
	cout << "============" << endl;
	cout << endl;

	cout << "Name				= " << mName << endl;
	cout << "Class				= " << mClassName << endl;
	cout << "Accuracy			= " << mAccuracy << endl;
	cout << "Hitpoints			= " << mHitPoints << endl;
	cout << "Max hitpoints		= " << mMaxHitPoints << endl;
	cout << "Xp					= " << mExpPoints << endl;
	cout << "Xp for next level	= " << mNextLevelExp << endl;
	cout << "Level				= " << mLevel << endl;
	cout << "Armor				= " << mArmor << endl;
	cout << "Weapon name		= " << mWeapon.mName << endl;
	cout << "Weapon damage		= " << mWeapon.mDamageRange.mLow << "-"
		 << mWeapon.mDamageRange.mLow << endl;
	cout << endl;
	cout << "END PLAYER STATS" << endl;
	cout << "================" << endl;
	cout << endl;
}

void Player::victory(int xp){
	cout << "You won the battle!" << endl;
	cout << "You win " << xp << " experience points!" << endl;
	mExpPoints += xp;
}

void Player::gameover(){
	cout << "You died in battle..." << endl << endl;
	cout << "=======================" << endl;
	cout << "GAME OVER!" << endl;
	cout << "=======================" << endl;
	cout << "Press 'q' to quit: " << endl;
	char q = 'q';
	cin  >> q;
	cout << endl;
}

void Player::displayHitPoints(){
	cout << mName << "'s hitpoints = " << mHitPoints << endl;
}

thanks

K

Last edited on

Computergeek01 (5613)

That all looks to be in order but do you ever actually define the class "Monster"?

hamsterman (4538)

You can pass a class& without defining it. What you can't do is access its members.

Krahl (63)

Yes the class Monster is defined in Monster.h.

Summing it up this is how it looked.

//Monster.h 
class Player;

class Monster{
public:
     void function(Player& player);
private:
     Player& player;
};

//Player.h 
class Monster;

class Player{
public:
     void function(Monster& monster);
private:
     Monster& monster;
};

//Player.cpp

#include "Player.h"

void function(Monster& monster){}

//Monster.cpp

#include "Monster.h"

void function(Player& player){}

I thought since in the method files the instances of the other classes appear as references only a forward definition is sufficient. I also thought that the forward definition would be taken from including the corresponding header files. But the problem was solved by including the .h files of the used classes explicitly. Not sure if I could explain it well. Any clarification on this matter?

@hamsterman
I understand. I think that was the problem.

Last edited on

Computergeek01 (5613)

None of your other classes headers seem to be including "Monster.h".

Krahl (63)

They don't need to include Monster.h the problem is solved by including "Monster.h" in the .cpp files only not the other header files. The .h's only need forward declarations and the .cpp's need the actual include file

Last edited on

Computergeek01 (5613)

If you had the "Monster.h" file in the headers you wouldn't need to forward declaration, that's pretty much what I wanted to point out

hamsterman (4538)

I also thought that the forward definition would be taken from including the corresponding header files.

It would be nice, but no. In c++ you have to state things explicitly. A & or * work because to pass them program doesn't need any information about the types they point to. Technically all * and &s are the same. Forward declaration is there to tell the compiler that Monster is actually a type and it doesn't need to throw the "undeclared identifier" error.

Krahl (63)

Thanks hamsterman and computergeek for your help.

Cheers

K

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