Another beginner question

This one is about object oriented programming. In java I could just add new files and have them be classes so if I wanted a Player class in java I would make a file that said:

Code:

---

public class Player
{
    public Player(int a, int b)
    {some stuff}

    --- some more methods
}

---

And in my driver I could say, Player p = new Player(1,2); or something.

In C++ I'm not really sure what to do...here's what I have so far and its giving me an error about /clr options or something:

Code:

---

#include <iostream>
#include <string>
using namespace std;

public class Player
{
  public:
    double points,value;
        string name, pos;
    Player (string,string,double);
};

Player::Player (string n, string p, double pts)
{
  name = n;
  pos = p;
  points = pts;
}

---

1) You don't declare a class in C as public, it's scope is determined by either where the class was defined, or if in a header, which class has included the header.

In C++ you don't have to dynamically allocate objects and have a reference like Java does by nature.

Your class would be delared as you have. In the main method or wherever the class is used it can be declared as such

Code:

---

Player a("abc", "def", 12.0); //Constructs objects a
Player* b = new Player("abc", "def", 12.0); //constructs an object dynamically as it would in java,
//b is only a pointer(reference) to a Playerobject

//object can then be used as such
*b.memberFunction(); //"dereferences" pointer, which means get the underlying object that b references
b->memberFunction(); //uses the arrow op, 'b->' acts as '*b.'

---

Change:

Code:

---

public class Player

---

To:

Code:

---

class Player

---

The public keyword used in this context is Java syntax, not C++ syntax.

Ok I did this, Player a(name, pos, ppoints); and I got a undeclared identifier error. Whasts the difference between doing Player a(stuff) and doing Player* a = new Player(stuff)

What is the exact error.

As to the difference between

Quote:

---

Player a(stuff) and doing Player* a = new Player(stuff)

---

Player a(stuff) means that you create a variable a, of the type Player.

Player *a = new Player(stuff) means that you care a variable a, that is a pointer to the type Player, which is then initialized to the memory location choosen by "new" for something of type Player.

In many cases, the difference isn't much at all - which is "better" really depends on how you want to use the variable - it is quite a complex matter to choose between the two, as they have all sorts of different benefits and drawbacks. The typical case where you want to have pointers is when you want to store a reference to this player elsewhere - whilst it's possible to do that even when you don't create them with new, it's the most natural choice in such a case.

--
Mats

Generally you want to use Player a(stuff) unless you have a good reason. Doing this allows the compiler to manage the lifetime of the object (it lasts until the end of the scope it is declared in).

Quote:

---

Originally Posted by Daved

Generally you want to use Player a(stuff) unless you have a good reason. Doing this allows the compiler to manage the lifetime of the object (it lasts until the end of the scope it is declared in).

---

Yes, I completely agree with that.

One of the biggest problems with modern software is that it "leaks memory", which means that if you run it for a long time, it uses more memory than it did in the beginning, without a change in the ACTUAL data being kept by the application. The cause for this is that the programmer did a "new" (or "malloc") without a corresponding "delete" (or "free"). Avoiding using pointers to things, and thus needing to use "new" is a good idea if it's possible (within reason - with experience you'll learn to know when you need one and when you need the other).

--
Mats

Quote:

---

Originally Posted by matsp

The typical case where you want to have pointers is when you want to store a reference to this player elsewhere - whilst it's possible to do that even when you don't create them with new, it's the most natural choice in such a case.

---

To expand upon this, here's how you would use a pointer and a reference without creating a variable with new:

Code:

---

class Player a("abc", "def", 12.0); //Constructs objects a
Player *ap = &a; //creates a pointer to type Player and puts the address of a in ap
ap->memberFunction();
Player &ar = a; //creates a reference and points it to a
ar.memberFunction();

---

Also, if you do use new, don't forget to use delete, or else you'll have a memory leak.

Code:

---

Player* b = new Player("abc", "def", 12.0); //constructs an object dynamically
delete b;

---

Yes, robwhit has it right - I was tempted to write a bit about that, as well as what DaveD commented on - but tried to keep it brief. This is obviously a forum where people comment when the initial post isn't complete :-)

--
Mats

the error just says error C2065: 'Player' : undeclared identifier. I don't have Player defined in this file though, its in Player.cpp like I would do in java

You need to have the class definition in a header file that you then #include in any source file that needs to work with a specific instance of this class:

class.h

Code:

---

class foo
{
    int bar;
public:
    foo();
};

---

class.cpp

Code:

---

#include "class.h"

foo::foo() : bar(0)
{
}

---

main.cpp

Code:

---

#include "class.h"

int main()
{
    foo myfoo;  // Create an instance of the class.

    ...

    return 0;
}

---

Ok so I have a Player.h, a Player.cpp, and a main file. I left out some unimportant code. I'm getting a Cannot open include file: 'Player.h': No such file or directory

Main:

Code:

---

#include <Player.h>
int main ()
{
Player a(name, pos, ppoints);
}

---

Class:

Code:

---

#include <Player.h>
Player::Player (string n, string p, double pts)
{
  name = n;
  pos = p;
  points = pts;
}

---

Header:

Code:

---

class Player
{
        double points,value;
        string name, pos;
        public:
                Player (string,string,double);
};

---

When you #include a file, if you use <>'s around the filename, the include directory of your compiler is searched. If you use ""'s instead, then it searches the current directory, etc. etc..

Ok I did "Player.h" instead of <Player.h> and now I have lots of errors:

Code:

---

class Player
{
        double points,value;
        string name, pos;
        public:
                Player (string,string,double);
};

---

Syntax error, missing ';' before identifier 'name'

The compiler doesn't know the type "string", so you need to include the support for that.
#include <string>

--
Mats

Your name data member is of type string, so in order for this to all work either:

1) The header needs to #include the string header, or...
2) Any source file that #includes this class header must also #include the string header before it #includes the class header.

You also need to be mindful of any namespace issues as well.

Maybe:

Code:

---

#include <string>

class Player
{
        double points,value;
        std::string name, pos;
        public:
                Player (std::string,std::string,double);
};

---

You also need header include guards. You also need more code in main to create variables named name, pos and ppoints. hk_mp5kpdw's code was an example, not meant to be used directly with your class and compiled immediately. You ahve more work to do.

I don't know what header guards are but I'm close...The variables are all defined in my main
class. I'm gettin an error error C2664: 'Player::Player(std::string,std::string,double)' : cannot convert parameter 3 from 'std::string' to 'double'

Player.cpp

Code:

---

#include <iostream>
#include <string>
#include "Player.h"
using namespace std;

Player::Player (string n, string p, double pts)
{
  name = n;
  pos = p;
  points = pts;
}

---

Player.h

Code:

---

#include <string>

using namespace std;

class Player
{
        double points,value;
        string name, pos;
        public:
                Player (string,string,double);
};

---

main

Code:

---

Player a(name, pos, ppoints);

---

that was a dumb error, nvm working now, i'll be back when it breaks again

>> I'm gettin an error error C2664
Apparently ppoints is a string variable, but you should be passing a double.

>> I don't know what header guards are but I'm close.
Look them up. You will need them to get this program working.

How do you properly provide implementation. Apparently this is wrong haha

Code:

---

#include <iostream>
#include <string>
#include "Player.h"
using namespace std;

Player::Player (string n, string p, double pts)
{
  name = n;
  pos = p;
  points = pts;
}

Player::void calcPoints()
{
        value = 10;
}

Player::double getPoints()
{
        return value;
}

---

Code:

---

#include <iostream>
#include <string>
#include "Player.h"
using namespace std;

Player::Player (string n, string p, double pts)
{
  name = n;
  pos = p;
  points = pts;
}

void Player::calcPoints()
{
    value = 10;
}

double Player::getPoints()
{
    return value;
}

---

ah sweet, ok, one(two) more errors. It says I need a class/struct/union

Code:

---

int main ()
{
  PlayerIO a();
  a.makeFile();
  a.makePlayerList();
  return 0;
}

---

Include guards is when you have a mechanism to prevent the same header-file having it's content included multiple times - most of the time, once is perfectly fine.

Example: myclass.h

Code:

---

#ifndef __MYCLASS_H__
#define __MYCLASS_H__

class myclass {
....
}
...
...
...
#endif

---

It is good practice to ALWAYS do this for ALL header-files, because somtimes (in larger projects) header-files are included by other header files. Let's say we have a "baseclass" that, that is then needed by classA and classB, and in our "main.c", we need both classA and classB, we want to include both "classA.h" and "classB.h". At the same time, we may not want to tell everyone using "classA" that they need to FIRST include "baseclass.h". So we add "baseclass.h" to both "classA.h" and "classB.h" - but without guarding against multiple includes, the content of "baseclass" would now be twice in the same compile - that's generally a bad idea. This is where include-guards are needed.

--
Mats

Note that identifiers with double underscores are reserved for the implementation, so it is best to not use them in your include guards. MYCLASS_H would be better.


>> It says I need a class/struct/union
What is PlayerIO? You have a class called Player, but the compiler doesn't recognize PlayerIO. Don't forget to post the entire text of the error message(s) to make it easier for us to help. Also, don't put empty prentheses after the variable when declaring it, the compiler will think it is a function declaration.

It is a function declaration...Error 1 error C2228: left of '.makeFile' must have class/struct/union c:\Documents and Settings\chyron\Desktop\jp\TestApp\TestApp\TestApp .cpp 10

I do have a PlayerIO class, I just made it. It has a function makeFile

And do you include the correct headerfile? Do you use the correct spelling everywhere?

--
Mats

yes, this is my main class. Its very short. I included both of the headers. They both exist

Code:

---

#include <string>
#include "Player.h"
#include "PlayerIO.h"
using namespace std;

int main ()
{
  PlayerIO a();
  a.makeFile();
  a.makePlayerList();
 
  return 0;
}

---

Didn't DaveD say something like "Don't put parenthesis behind the variable name, the compiler will think it's a function"? I clearly seem to remember that. Read your code again...

--
Mats

>> Didn't DaveD say something like "Don't put parenthesis behind the variable name, the compiler will think it's a function"?
I added that as an edit, sorry if it was too late to be seen.

But they are functions...thats what I'm saying...except PlayerIO a(), thats me attempting to construct an object. You had said to construct an Object it was ObjectType o(parameters); but if I have no parameters shouldn't it just be ObjectType o();

Yes, that's one of the interesting inconistencies that came out of trying to retrofit object orientation into an already existing language.

How would you declare a function that takes no parameters that return an object of ClassA? If we assume this is:

Code:

---

ClassA func();

---

then how do you expect the compiler to tell the difference from that and the "no parameters" construction, constructing a variable called func?

Since one form already existed originally in C (the former), the latter had to be the one that differs. [A bit like the joke about the ship sending a message to the lighthouse to "move to your left" - it ain't no good trying to move something solid!]

--
Mats

Ok well I took the parenthesis away and it worked, now to just clean up the rest of the program.

I'm doing a lot of substring work, which is kind of annoying because it seems to not work the way I want it. For example, I have a line of text that looks like this:

Code:

---

      Nick Swisher</td>      1B/0F</td>      266.9</td>

---

After some annoying substring manipulation I get it down to this:

Code:

---

1B/0F</td>      266.9</td>

---

I need to be able to realize that this line of text contains 1B and 0F so I tried this:

Code:

---

int secpos = line.find("/");
int endpos = line.find("</td>");
        if(secpos < endpos)//more than one pos
        {
                pos1 = line.substr(0,secpos);
                pos2 = line.substr(secpos+1,endpos);
        }
        else pos1 = line.substr(0,endpos);

---

If the / comes before the </td> it means I have more than one position, otherwise it'll pick up the / in </td> and there will only be one position. However,when I want to print out the positions it doesn't give me what I want. This line fore example prints "1B and 0F</t"

I think I'm using substr wrong. I was thinking it'd be easier to just replace all of the occurences of </td> with something but I'm not sure how to do that either

You are using substr wrong - the second parameter is the length, not the position to end at...

--
Mats

Ok, that makes sense...does C++ have arraylists or lists of any kind like Java does?

Yes, the standard library has a bunch of containers. vector is a dynamic array, deque is a double-ended queue, list is a doubly linked list, map is a sorted map, set is a sorted map where the key and value are a single object, multimap and multiset are versions of map and set that allow duplicates. I might be forgetting some. There are also adapters that confine the behavior of one of the ones above to a specific interface, like stack or queue. There are not-quite standard containers called unordered_map, unordered_set, unordered_multimap and unordered_multiset that are not sorted and use a hash based implementation.

For many needs, a vector is a good start. I'd try that first.

I wanted to make the vector a variable in the headerfile for PlayerIO but then I have to include Player.h in PlayerIO.h, and then I get a lot of errors. Is this the best way to do that or should vectors not go into headerfiles

This is how it's currently laid out, with comments to show how I want it to be laid out

PlayerIO.h

Code:

---

#include <vector>
#include "Player.h"
using namespace std;

class PlayerIO
{
  vector<Player> allPlayers;
        public:
                PlayerIO();
                void makePlayerList();
                void sortList();
};

---

PlayerIO.cpp

Code:

---

#include <vector>
#include "PlayerIO.h"
#include "Player.h"
using namespace std;

PlayerIO::PlayerIO()
{

}

void PlayerIO::makePlayerList()
{
  Player a(stuff);
  allPlayers.push_back(a);
}

void PlayerIO::sortList()
{
  for(int i = 0; i<allPlayers.length; i++)
        stuff;
}

---

So, I take it the errors/warnings are simply because you include "player.h" twice? You need include-guards, as discussed previously in this thread.

--
Mats

Error 1 error C2011: 'Player' : 'class' type redefinition

When I actually make the changes I wanted to make (I'll edit that code) I get this error.
I don't understand, the guards are so I don't include it more than once? But don't I need to because differetn classes need to use it? PlayerIO.cpp needs to include Player.h so it can add Players to a list of Players. PlayerIO.h needs to include Player.h so it can make an actual vector of Players.

EDIT: OH! PlayerIO.cpp includes PlayerIO.h and Player.h but PlayerIO.h already includes Player.h...so I just didn't include Player.h in PlayerIO.cpp...problem solved

Quote:

---

Originally Posted by jcafaro10

OH! PlayerIO.cpp includes PlayerIO.h and Player.h but PlayerIO.h already includes Player.h...so I just didn't include Player.h in PlayerIO.cpp...problem solved

---

Yes, but the CORRECT way to solve that problem is to have include-guards so that the include file is taken only once, whether you include it once or a hundred times. Consider that somone may not KNOW that you shouldn't include player.h when using playerIO.h. Or someone writes a piece of code that uses player, then needs playerIO too, and adds playerIO.h to the .cpp file, and suddenly all h**l breaks loose in the form of errors and warnings.

--
Mats