Thread: Class help (understanding)

Ok, im working on classes (since i abandoned my MUD codebase until i can actualy use C++) but im a tiny bit confused on the constructor and destructor... i dont know how to add arguments yet so im not worried about that on the constructor, but does the program automatically call those functions? or do i have to put something to stop them myself? here is an example i just made using them

Code:

#include <iostream>

using namespace std;

class Montez {
	public:
		Montez();
		~Montez();
		void talk();
		int number;
};

Montez::Montez() {

}

Montez::~Montez() {
}

void talk(Montez &M) {
	cout<<"TADA! it works! "<<endl;
	cout<<"And, your number was "<<M.number<<". "<<endl;
}

int main() {
	Montez Monte;

	cout<<"Enter a number: ";
	cin>>Monte.number;
	cin.get();
	talk(Monte);
}

i also did some practice on using functions to call structures and it apparently works for classes too... so is there a way i have to call the destructor or constructor? or is there anywere in that (small) code that i messed up on? thanks for the help C++ is starting to get easier...

Constructors and destructors are never called until an object is created. Adding parameters to contstructors and destructors is the same as it is for regular functions. When you create an object, the constructor is called. It's often used to initialize variables in the class. A destructor is called when an object is deleted. Try this example code:

Code:

#include <iostream>
using std::cout;
using std::cin;

class Cat{
   private:
     int itsAge;
   public:
     int GetAge(){return itsAge;}
     Cat(int age);
     ~Cat();
};

Cat::Cat(int age){
    cout << "Hey, you created a cat!\n";
    itsAge=age;
}
Cat::~Cat(){
    cout << "Cat object deleted.\n";
}

int main(void){
/*Create a pointer to a Cat named Frisky; 
pass 5 for a parameter to the constructor.*/
    Cat *Frisky = new Cat(5);
    cout << "The cat is "<<Frisky->GetAge()<< " years old.\n";
    delete Frisky; //Bye bye, kitty!
    cin.get();
    return 0;
}

so it would be easier to put the classes in a header file and just not use structures? or can structures be used in header files too? i havent seem any like that yet... (BTW ive never made my own header file, im just guessing from looking at other headers...)

Ok, here is another one, but im getting errors (i know its trying to call the class in the function but im not getting it... i called it exactly like i did the last one...)

Code:

#include <iostream>

using namespace std;

class Person {
	public:
		Person();
		~Person();
		string name;
		int age;
		string hobby;
		int siblings;
		void results();
};

Person::Person() {
}

Person::~Person() {
}

int main() {
	Person Perso;
	
	cout<<"Whats your name? ";
	cin>>Perso.name;
	cin.get();
	cout<<"How old are you? ";
	cin>>Perso.age;
	cin.get();
	cout<<"Whats your hobby? ";
	cin>>Perso.hobby;
	cin.get();
	cout<<"How many siblings do you have? ";
	cin>>Perso.siblings;
	cin.get();
	results(Person);
}

void results(Person &P) {
	cout<<"Your name is: "<<P.name<<". "<<endl;
	cout<<"You are "<<P.age<<" years old. "<<endl;
	cout<<"Your hobby is "<<P.hobby<<". "<<endl;
	cout<<"You have "<<P.siblings<<" siblings. "<<endl;
}

Originally Posted by Goosie

so it would be easier to put the classes in a header file and just not use structures? or can structures be used in header files too?

Classes and structures are identical in C++ except:
a) The default access specifier for a class is private.
b) The default access specifier for a struct is public.

Which gives us:

Code:

class Foo
{
    int privatevariable;
};

struct Bar
{
    int publicvariable;
};

Other than that, they're identical.


Quzah.

Originally Posted by Goosie

Code:

class Person {
	public:
		Person();
		~Person();
		string name;
		int age;
		string hobby;
		int siblings;
		void results();
};

...snip...

	results(Person);
}

void results(Person &P) {
	cout<<"Your name is: "<<P.name<<". "<<endl;
	cout<<"You are "<<P.age<<" years old. "<<endl;
	cout<<"Your hobby is "<<P.hobby<<". "<<endl;
	cout<<"You have "<<P.siblings<<" siblings. "<<endl;
}

For starters, you're saying that your class has a member function (method if you will) called 'results'. However, you don't actually make said method for it. Next you declare a function and try to use it, but you haven't prototyped it any place.

Assuming you actually wanted 'results' to be a method of your class, it would go something like this:

Code:

Person::results( )
{
    cout<<"Your name is: "<<name<<". "<<endl;
    cout<<"You are "<<age<<" years old. "<<endl;
    cout<<"Your hobby is "<<hobby<<". "<<endl;
    cout<<"You have "<<siblings<<" siblings. "<<endl;
}

...

Perso.results(); //call this method of this class instance 'Perso'

Quzah.
}

^didnt work, so I tried taking it out of the class and i still get that error... hhmmm...

oh yea, this is the error i get... sorry, i forgot to post it...

Code:

[montez@localhost montez]$ c++ class.cpp -o class -O2 -Wall
class.cpp: In function `int main()':
class.cpp:5: error: too many arguments to function `void results()'
class.cpp:40: error: at this point in file

EDIT:
oh yea, and thats after i changed the code to this:

Code:

#include <iostream>

using namespace std;

void results();

class Person {
	public:
		Person();
		~Person();
		string name;
		int age;
		string hobby;
		int siblings;
};

Person::Person() {
	cout<<"Constructor started... "<<endl;
}

Person::~Person() {
	cout<<"YOU KILLED IT!!! "<<endl;
}

int main() {
	Person Perso;
	
	cout<<"Whats your name? ";
	cin>>Perso.name;
	cin.get();
	cout<<"How old are you? ";
	cin>>Perso.age;
	cin.get();
	cout<<"Whats your hobby? ";
	cin>>Perso.hobby;
	cin.get();
	cout<<"How many siblings do you have? ";
	cin>>Perso.siblings;
	cin.get();
	results(Perso);
}

void results(Person &P) {
	cout<<"Your name is: "<<P.name<<". "<<endl;
	cout<<"You are "<<P.age<<" years old. "<<endl;
	cout<<"Your hobby is "<<P.hobby<<". "<<endl;
	cout<<"You have "<<P.siblings<<" siblings. "<<endl;
}

I think that part of your problem is with the results function. The function decleration and the function implementation need to be identical. You have the decleration at the top showing no parameters, while the implementation has a Person parameter.

Also, you may need #include <string>? not sure though

Code:

#include <iostream>

using namespace std;

void results(Person&);

I tried added Person &P in the parameters on the top and it gave a lot more errors than i started with...

EDIT:
yours didnt work either stoned... functions seem to really hate me...

EDIT AGAIN:
quizkiwi, the string header isnt needed... (i added it to check)... thanks for trying tho everyone...

Ok, i got it to work, so i saved the copy giving me problems and im going to google it and try to figure out why its not working... but on another copy i made it all 1 function and it works perfectly... thanks for the help everyone!

Code:

#include <iostream>

using namespace std;

class Person {
	public:
		Person();
		~Person();
		string name;
		int age;
		string hobby;
		int siblings;
};

Person::Person() {
	cout<<"Constructor started... "<<endl;
}

Person::~Person() {
	cout<<"YOU KILLED IT!!! "<<endl;
}

int main() {
	Person Perso;

	cout<<"Whats your name? ";
	cin>>Perso.name;
	cin.get();
	cout<<"How old are you? ";
	cin>>Perso.age;
	cin.get();
	cout<<"Whats your hobby? ";
	cin>>Perso.hobby;
	cin.get();
	cout<<"How many siblings do you have? ";
	cin>>Perso.siblings;
	cin.get();
	cout<<endl<<endl<<endl<<endl;
	
	cout<<"Your name is: "<<Perso.name<<". "<<endl;
	cout<<"You are "<<Perso.age<<" years old. "<<endl;
	cout<<"Your hobby is "<<Perso.hobby<<". "<<endl;
	cout<<"You have "<<Perso.siblings<<" siblings. "<<endl;
}

So in other words, exactly what I said was wrong, that you ignored. Rather than fix the function to work correctly, you just threw it out the window.


Quzah.