I am just started learning OOP concepts and to help myself learning, I have created a Characters class. From this class I have made instance called main and an instance called monster. Here is the code for the class:
#include <iostream>
#include <string>
using namespace std;
class Character {
public:
string name;
float health;
int attackLevel;
int defenseLevel;
void setAttr(string sName,float sHealth, int sAttackLevel, int sDefenseLevel) {
name = sName;
health = sHealth;
attackLevel = sAttackLevel;
defenseLevel = sDefenseLevel;
}
void attack(int whatInstanceToAttack) {
whatInstanceToAttack.hitpoints -= 20; //obviously not valid but how do i do this?
return whatInstanceToAttack;
}
int defend(string defend) {
int damageRelieved = defenseLevel * 2;
return damageRelieved;
}
};
int main() {
Character main;
Character monster;
main.setAttr("Rafael",200,100,30);
monster.setAttr("Monster1",30,40,30);
cout << "Default Values for Raf are;" << endl;
cout << main.name << endl;
cout << main.health<< endl;
cout << main.attackLevel << endl;
cout << main.defenseLevel << endl;
cout << "Default values for monster are" << endl;
cout <<monster.name << endl;
cout <<monster.health << endl;
cout << monster.attackLevel<< endl;
cout << monster.defenseLevel << endl;
return 0;
}
Basically what I want to do is somehow access the monster instance via the main instance. I want to do this by running the attack method. So if I run
main.attack(monster);
then I want the monster to lose 20 hitpoints.
How do I go about doing this?
All you need is to pass reference of Character in attack method.
I think you must be aware of pass by value and pass by reference concept. If not you can read it here
void attack(Character &whatInstanceToAttack) {
whatInstanceToAttack.hitpoints -= 20; //obviously not valid but how do i do this?
}
Yes you can access the variables of an instance from another instance of the same class. You need to use a reference to the object to ensure the changes are reflected in the other instance. So here is what your attack function should look like.
void attack(Character &c)
{
c.hitpoints - = 20;
}
Now when you call main.attack(monster) from the main() function, the hitpoints of monster will get decremented by 20.
As a side note, it is considered a good practice to make the data members of a class private, to avoid illegal access/modification of the data. Always use the member functions as an interface to your class instances.
overload the method attack and you can pass by value or reference as per your requirement.
void attack(Character chr)
or
void attack(Character &chr)
Related
I have the following problem: I wanted to redo a project from good old C to C++ and make everything class(y) :) and keep it scalable from the beginning.
It is a simulation of cells (being part of a swarm) on a grid, so I decided the following structure:
class Simulation has an instance of
class Grid has an instance of
class Swarm has an instance of
class Cell
I defined the classes in separate header files. Then I need, of course, to be able to call functions in grid, swarm and cell as well. I wanted to do it straight forward:
Simulation mysim;
mysim.get_grid(0).any_function_here();
with the grid as return parameter
Grid Sim::get_grid(int grid_no)
{
std::cout << "sim.get_grid(" << grid_no << ") called." << std::endl;
if (grid_no <= amount_of_grids)
return this->test;//##//this->gridlist[grid_no];
else
std::cout << "you have not created this grid number yet" << std::endl;
Grid dummy;
return dummy;
}
It calls the function and works as long as no changes in the grid are made. These seem to be lost in space. Probably a pointer error, but I cannot find an error, since exactly the same code is working for the Simulation class...
More source:
int Grid::create_swarm(std::string name)
{
Swarm new_swarm;
new_swarm.set_name("Protoswarm");
swarmlist.push_back(new_swarm);
this->amount_of_swarms ++;
std::cout << "amount_of_swarms = " << amount_of_swarms << std::endl;
return 0;
}
Swarm Grid::get_swarm(int swarm_no)
{
std::cout << "grid.get_swarm(" << swarm_no << ") called." << std::endl;
if (swarm_no <= amount_of_swarms)
return swarmlist[swarm_no];
else
std::cout << "oh oh - you have not this swarm in here..." << std::endl;
Swarm dummy;
return dummy;
}
I can call the create_swarm function as often as I want, but the swarms do never appear and the counter does not raise in that grid, just temporarily as long as the funtion is in there. Am I missing something? Is it really just a pointer error? Why does this code work if I call it like this:
Grid newgrid;
newgrid.create_swarm();
A quickly c&p'ed MWE
#include <iostream>
#include <string>
#include <vector>
class Sim
{
public:
Sim();
virtual ~Sim();
Grid get_grid(int grid_no);
protected:
private:
std::vector<Grid> gridlist;
int amount_of_grids = -1;
};
class Grid
{
public:
Grid();
virtual ~Grid();
int set_size(int x, int y);
int create_swarm(std::string name);
Swarm get_swarm(int swarm_no);
void print_swarms();
protected:
private:
std::vector<Swarm> swarmlist;
int amount_of_swarms = -1;
/*static const*/ int size_x;
/*static const*/ int size_y;
std::vector<std::vector<Field>> fields;
std::string gridname;
};
Grid Sim::get_grid(int grid_no)
{
std::cout << "sim.get_grid(" << grid_no << ") called." << std::endl;
if (grid_no <= amount_of_grids)
return this->gridlist[grid_no];
else
std::cout << "you have not created this grid number yet" << std::endl;
Grid dummy;
return dummy;
}
int Grid::create_swarm(std::string name)
{
Swarm new_swarm;
new_swarm.set_name("Protoswarm");
swarmlist.push_back(new_swarm);
this->amount_of_swarms ++;
std::cout << "amount_of_swarms = " << amount_of_swarms << std::endl;
return 0;
}
Swarm Grid::get_swarm(int swarm_no)
{
std::cout << "grid.get_swarm(" << swarm_no << ") called." << std::endl;
if (swarm_no <= amount_of_swarms)
return swarmlist[swarm_no];
else
std::cout << "oh oh - you have not this swarm in here..." << std::endl;
Swarm dummy;
return dummy;
}
using namespace std;
int main(int argc, char* argv[])
{
Sim mysim;
mysim.create_grid();
mysim.get_grid(0).create_swarm("Alpha-Swarm");
mysim.get_grid(0).create_swarm("Betaa-Swarm"); //doesn't work
Grid newgrid;
newgrid.create_swarm("Gamma-Swarm");
newgrid.create_swarm("Delta-Swarm"); // works, but is not needed.
return 0;
}
Grid Sim::get_grid(int grid_no) {...}
You are returning by value, not by reference. That means that what you are returning is a copy of your actual member. In your case, however, you want to be returning by reference in order to be able to make changes to the original object. Your code would become
Grid& Sim::get_grid(int grid_no) {...}
Keep in mind, however, that you will not be able to return any temporaries that way (such as your dummy Grid), so you will need to change your methods to circumvent this issue. If you do not want to do this, you could still return a pointer, although this would change the syntax a little.
Your get_grid and get_swarm methods return copies of original array items. You should return reference (or a pointer) to Grid or Swarm instead.
I'm a beginner in C++ and yesterday i did my first step with the OO (Object Oriented).
I made a programm that has a character class (Personnage), i made three variables for the character, vie, mana and degat.
I made also a function in the class which is attack and it should remove health from john, but when i show the health of john with another function i see 100, can someone help me please ?
#include <iostream>
#include <string>
using namespace std;
class Personnage
{
public:
void montrer()
{
cout << vie << endl;
}
void attaquer(Personnage john)
{
vie = vie - degat;
if (vie < 0)
{
vie = 0;
}
}
private:
int degat = 50;
int vie = 100;
int mana = 100;
};
int main
{
int action(0), degat;
Personnage jack, john;
cout << "What action do you want to do ?" << endl;
cout << "1-Attack your ennemy." << endl;
cout << "2-Take some life" << endl;
cout << "3-Take some mana." << endl;
cin >> action;
switch(action)
case '1':
jack.attaquer(john);
john.montrer();
return 0;
}
First, you must do "case 1" instead of " case '1' " because your variable is integer. However, this wont solve your problem, and you will still see john's state unaffected by the call of the method "attaquer". Why? because method attaquer is invoked on the personnage Jack, not John. If you want the method to change the health of the attacked Paersonnage, not the attacking one, you should make it like this:
void attaquer(Personnage& victime) // notice the &, pass by reference
{
victime.vie -= degat;
if (victime.vie < 0)
{
victime.vie = 0;
}
}
void attaquer(Personnage john)
{
vie = vie - degat;
if (vie < 0)
{
vie = 0;
}
}
vie and degat both refer to the fields of the object the method was called from. You call attaquer from jack, so jack's data changes. You then call john.montrer(), which will show john's untarnished data.
I assume you intended that attaquer affect the Personnage passed to it. To alter the passed parameter:
void attaquer(Personnage john)
{
john.vie = john.vie - degat;
if (john.vie < 0)
{
john.vie = 0;
}
}
You have to specify that you want to use a different object's data. This, however, is not a complete solution because void attaquer(Personnage john) takes a Personnage by value. This means it copies the object you give it, and that new copy disappears at the end of the function. Make it a reference like:
void attaquer(Personnage& john)
This will affect the original object you sent to the function.
Of course, this answer points out that your switch statement is comparing an integer to a character 1 and '1' are two different values.
I've been pulling my hair out trying to figure out this program. The class has to hold 3 player's info and output their info. My output function is not outputting from my set/get functions. Also, if I output the array indexes the program crashes (that's the array indexes are commented out in the Output function).
edit: I'll just show one profile to keep the code smaller
Any help is appreciated.
#include <cstdlib>
#include <iostream>
using namespace std;
class PlayerProfile
{
public:
void output();
void setName1(string newName1); //player's name
void setPass1(string newPass1); //player's password
void setExp1(int newExp1); //player's experience
void setInv1(string newInv1[]); //player's inventory
void setPos1(int newX1, int newY1); //player's position
string getName1();
string getPass1();
int getExp1();
string getInv1();
int getPos1();
private:
string name1;
string pass1;
int exp1;
string inv1[];
int x1;
int y1;
};
int main(int argc, char *argv[])
{
PlayerProfile player;
cout << "This program generates three player objects and displays them." << endl;
cout << endl;
player.output();
system("PAUSE");
return EXIT_SUCCESS;
}
void PlayerProfile::setName1(string newName1)
{
newName1 = "Nematocyst";
name1 = newName1;
}
void PlayerProfile::setPass1(string newPass1)
{
newPass1 = "obfuscator";
pass1 = newPass1;
}
void PlayerProfile::setExp1(int newExp1)
{
newExp1 = 1098;
exp1 = newExp1;
}
void PlayerProfile::setInv1(string newInv1[])
{
newInv1[0] = "sword";
newInv1[1] = "shield";
newInv1[2] = "food";
newInv1[3] = "potion";
inv1[0] = newInv1[0];
inv1[1] = newInv1[1];
inv1[2] = newInv1[2];
inv1[3] = newInv1[3];
}
void PlayerProfile::setPos1(int newX1, int newY1)
{
newX1 = 55689;
x1 = newX1;
newY1 = 76453;
y1 = newY1;
}
string PlayerProfile::getName1()
{
return name1;
}
string PlayerProfile::getPass1()
{
return pass1;
}
int PlayerProfile::getExp1()
{
return exp1;
}
string PlayerProfile::getInv1()
{
return inv1[0], inv1[1], inv1[2], inv1[3];
}
int PlayerProfile::getPos1()
{
return x1, y1;
}
void PlayerProfile::output()
{
cout << "Player Info - " << endl;
cout << "Name: " << name1 << endl;
cout << "Password: " << pass1 << endl;
cout << "Experience: " << exp1 << endl;
cout << "Position: " << x1 << ", " << y1 << endl;
cout << "Inventory: " << endl;
/*cout << inv1[0] << endl;
cout << inv1[1] << endl;
cout << inv1[2] << endl;
cout << inv1[3] << endl; */
}
This is the output that I am getting:
This program generates three player objects and displays them.
Player Info -
Name:
Password:
Experience: -2
Position: 3353072, 1970319841
Inventory:
Press any key to continue . . .
I'm sorry if I sound like an idiot, this is the first time I have programmed with classes and I am very confused.
First:
You do not have a constructor declared or defined in your class so when you compile, the compiler provides you with a default constructor.
The line
PlayerProfile player;
calls the default constructor provided by the compiler. This default constructor only allocates memory for your class member variables, but does not set their values. This is why name1, pass1, exp1, x1, y1 are not outputting what you expect.
Second:
C++ will not call get or set functions for you, and I think you are misunderstanding how c++ functions work.
this
void PlayerProfile::setName1(string newName1)
{
name1 = newName1;
}
is a function definition. You do not need to assign newName1 inside the function. It's value is passed to the function when a line like
setName1("Nematocyst");
is executed.
If you write a constructor, you can use it to call your set functions, and pass them the values you want to set member variables to.
If you do not want to write a constructor, you can call class functions/methods from main with:
player.setName1("Nematocyst");
Third:
Your program crashes because you are not using arrays properly. Here is a tutorial on how to declare an array and access it's contents.
Generally, I think you are trying to run before you know how to walk. Try not to get frustrated. Learn how arrays work, how functions work, and then how classes work. I hope this is not your homework assignment!
I am still wrapping my head around classes and am still new to C++. My assignment is:
Create three small classes unrelated by inheritance - classes
Building, Car and Bicycle. Give each class some unique appropriate
attributes and behaviors that it does not have in common with other
classes.
Write an abstract class CarbonFootprint with only a pure virtual
getCarbonFootprint method.
Have each of your classes inherit from that abstract class and
implement the getCarbonFootprint method to calculate an appropriate
carbon footprint for that class (check out a few websites that explain
how to calculate carbon footprints).
Write an application that creates objects of each of the three
classes, places pointers to those objects in a vector of
CarbonFootprint pointers, then iterates through the vector,
polymorphically invoking each object's getCarbonFootprint method.
For each object, print some identifying information and the object's
carbon footprint.
I am having trouble trying to figure out how to iterate through my vector <CarbonFootPrint>. I also do not know if the objects being created are actually being put into this vector. My code so far is:
#include <iostream>
#include <vector>
using namespace std;
class CarbonFootPrint
{
//class declarations
public:
virtual double getCarbonFootPrint();
};
//class implementation
double CarbonFootPrint::getCarbonFootPrint()
{
return 0;
}
class Building : CarbonFootPrint
{
//class declarations
public:
Building(double e = 0, int m = 12); //constructor
~Building(); //destructor
double setElectric();
virtual double getCarbonFootPrint();
private:
double electric;
int months;
};
//class implementation
Building::Building(double e, int m)
{
electric = e;
months = m;
}
Building::~Building()
{
}
double Building::setElectric()
{
cout << "Enter your monthly electric in KWH: " << endl;
cin >> electric;
return electric;
}
double Building::getCarbonFootPrint()
{
//I would like to print out the variable information for each object created
//and then
cout << "The carbon footprint for this house is " << endl;
//when it iterates through the vector.
return(electric * months);
}
class Car : CarbonFootPrint
{
public:
Car(double = 0, double = 0); //constructor
~Car(); //destructor
double setYearlyMiles();
double setAverageMPG();
virtual double getCarbonFootPrint();
private:
double yearlyMiles, averageMPG;
int co2 = 9;
};
//class implementation
Car::Car(double ym, double mpg)
{
yearlyMiles = ym;
averageMPG = mpg;
}
Car::~Car()
{
}
double Car::setYearlyMiles()
{
cout << "Enter in your yearly miles: " << endl;
cin >> yearlyMiles;
return yearlyMiles;
}
double Car::setAverageMPG()
{
cout << "Enter in your average miles per gallon: " << endl;
cin >> averageMPG;
return averageMPG;
}
double Car::getCarbonFootPrint()
{
//I would like to print out the variable information for each object created
//and then
cout << "The carbon footprint for this car is " << endl;
//when it iterates through the vector.
return((yearlyMiles * averageMPG) * co2);
}
class Bicycle : CarbonFootPrint
{
public:
Bicycle(double = 0, int = 34); //constructor
~Bicycle(); //destructor
double setMiles();
virtual double getCarbonFootPrint();
private:
int calories;
double miles;
};
//class implementation
Bicycle::Bicycle(double m, int c)
{
miles = m;
calories = c;
}
Bicycle::~Bicycle()
{
}
double Bicycle::setMiles()
{
cout << "Enter in number of miles: " << endl;
cin >> miles;
return miles;
}
double Bicycle::getCarbonFootPrint()
{
//I would like to print out the variable information for each object created
//and then
cout << "The carbon footprint for this bicycle is " << endl;
//when it iterates through the vector.
return (miles * calories);
}
Here is my main program:
int main()
{
vector <CarbonFootPrint> *list;
int answer, i;
cout << "Welcome to the Carbon Footprint Calculator!\n" << endl;
do
{
cout << "Main Menu\n" << endl;
cout << "1: Set house info.\n" << endl;
cout << "2: Set car info.\n" << endl;
cout << "3: Set bicycle info.\n" << endl;
cout << "4: Get carbon footprint for all items set.\n" << endl;
cin >> answer;
switch (answer)
{
case 1:
{
cout << "\n" << endl;
Building *anotherBuilding;
anotherBuilding = new Building;
anotherBuilding->setElectric();
cout << "\n" << endl;
break;
}
case 2:
{
cout << "\n" << endl;
Car *anotherCar;
anotherCar = new Car;
anotherCar->setYearlyMiles();
anotherCar->setAverageMPG();
cout << "\n" << endl;
break;
}
case 3:
{
cout << "\n" << endl;
Bicycle *anotherbike;
anotherbike = new Bicycle;
anotherbike->setMiles();
cout << "\n" << endl;
break;
}
case 4:
{
//have it iterate through the vector and print out each carbon footprint.
break;
}
default:
{
cout << answer << " is not a valid option" << endl;
break;
}
}
}
while (answer != 4);
system("pause");
return 0;
}
Any help or guidance is greatly appreciated! Thank you for your time!
Re: your comments on R Sahu's post (I'm too new to be allowed to comment on other posts)
You cannot access the base class because it has defaulted to private,
class Building : CarbonFootPrint
class Car : CarbonFootPrint
class Bicycle : CarbonFootPrint
are all inheriting from CarbonFootPrint privately, which represents a "has-a" relationship, in this case although semantically a car has-a carbon foot print you're actually trying to make an "is-a" relationship, as these are all objects that implement the base class, possibly a better name for CarbonFootPrint could be CarbonFootPrintProducer.
The fix here is simply make them all publically inherited
class Name : public Base
You missed a crucial thing from:
Write an application that creates objects of each of the three classes, places pointers to those objects in a vector of CarbonFootprint pointers, then iterates through the vector, polymorphically invoking each object's getCarbonFootprint method.
Instead of
vector <CarbonFootPrint> *list; // Pointer to a vector of CarbonFootPrint objects.
You need to use
vector <CarbonFootPrint*> list; // A vector of CarbonFootPrint pointers.
Instead of
{
cout << "\n" << endl;
Building *anotherBuilding;
anotherBuilding = new Building;
anotherBuilding->setElectric();
cout << "\n" << endl;
break;
}
Use
{
cout << "\n" << endl;
Building *anotherBuilding;
anotherBuilding = new Building;
anotherBuilding->setElectric();
// Add the pointer to the list of pointers.
list.push_back(anotherBuilding);
cout << "\n" << endl;
break;
}
Make similar changes to the other object types.
At the end, call CarbonFootPrint() on the objects:
for (auto item : list )
{
item->CarbonFootPrint();
}
and delete the objects:
for (auto item : list )
{
delete item;
}
R Sahu covered almost all of it, the only bit I see remaining is;
Write an abstract class CarbonFootprint with only a pure virtual getCarbonFootprint method.
to which your class is not abstract as the function is not pure virtual, to do this simply add = 0
virtual double getCarbonFootPrint() = 0;
Pure virtual functions are ones that don't count as implemented, any class containing a pure virtual function is called "abstract" and cannot be instantiated. In this case it would have helped you identify your vector of CarbonFootPrint rather than CarbonFootPrint* to them, as the instantiations would be picked up by the compiler, although knowing template compiler errors you'd have had a couple hundred lines of error message for this simple mistake.
For full disclosure: a pure virtual function can actually be defined, e.g.;
virtual double getCarbonFootPrint() = 0;
double CarbonFootPrint::getCarbonFootPrint()
{
// some code
return 0;
}
which can then be called from any derived class
double Building::getCarbonFootPrint()
{
return CarbonFootPrint::getCarbonFootPrint();
}
This is legal C++, allows you to define a default implementation, and still CarbonFootprint is an abstract class.
I'm trying to figure out how I can or why I can't access the member of this class. First I'll show you what works so you know what I'm thinking, then I'll show you what I can't seem to do.
What I can do is this: I have a class with a member. I make an pointer array of that class and make NEW pieces of it (through loop) and that's fine and all. I can also make another class with a similar array and even make NEW instances of that as well and initialize them, but when I try to access them, I have problems.
This code almost works fine:
#include <iostream>
using namespace std;
class testClass{
public:
int number;
};
class testPoint{
public:
testClass testInstance;
testClass *testclassArray[5];
void makeArray();
void setToI();
};
void testPoint::makeArray(){
for (int i = 0; i < 5; i++){
testclassArray[i] = new testClass;
}
}
void testPoint::setToI(){
for (int i = 0; i < 5; i++){
(*testclassArray[i]).number = i;
}
}
int main(void){
testPoint firstTestPoint;
firstTestPoint.makeArray();
firstTestPoint.setToI();
// EXCEPT FOR THIS LINE this is where I have problems
cout << firstTestPoint.(*testclassArray[0]).number << endl;
return 0;
}
I know this should work becuase this works
int main(void){
testPoint firstInstance;
firstInstance.testInstance.number = 3;
cout << firstInstance.testInstance.number << endl;
// and this works
return 0;
}
and this works
int main(void){
testClass *testPointer[5];
for (int i = 0; i < 5; i++){
testPointer[i] = new testClass;
(*testPointer[i]).number = i;
}
cout << (*testPointer[0]).number << endl;
return 0;
}
so why can't I access the members on the cout function the same way?
The following is invalid syntax:
cout << firstTestPoint.(*testclassArray[0]).number << endl;
The most common way to write what you are trying to accomplish is:
cout << firstTestPoint.testclassArray[0]->number << endl;
But, if you prefer, you can also write:
cout << (*firstTestPoint.testclassArray[0]).number << endl;
(The second way is far less common.)
The . operator is used to access members of direct objects, e.g. a.member where a might be declared as struct A a;. The -> operator is used to access members of indirect objects (aka pointers to objects), e.g. b->member where b might be declared as struct B* b = new B();.
You are dereferencing the variable in an incorrect way.
Try doing
cout << firstTestPoint.testclassArray[0]->number << endl;
instead.
In the same way the second attempt, where it works for you, could also have been written:
out << testPointer[0]->number << endl;
Try using this code:
cout << firstTestPoint.testclassArray[0]->number << endl;