Programming Forums > Application Development > C++

aznluvsmc · Sep 27th, 2005, 7:54 PM

Let's assume I have the following code with the data members having relevant data.

class book {
  int price;
  char title[21];

  public:
  int comparePrice(book);
};

int main() {

  book a1, b1; //assume a constructor set the variables to something

  a1.comparePrice(b1);
 
  return 0;
}

int book::comparePrice(book z) //returns 0 or 1
{
   int rv;  
   if (price < z.price)
       rv = 0;
   else
      rv = 1;

  return rv;
}

My question is since comparePrice is a member function I know it can access the object's price directly but doesn't the member function also become associated with a particular object of the class? What I mean is, how come my comparePrice() function call can access the private member of the passed book object if the function is a member of the a1 object?

I hope the question is clear.

This was an example from my class but I didn't have time to ask that question .

DaWei · Sep 27th, 2005, 8:00 PM

a1.comparePrice uses the a1 price; you just told it to. It compares it against the b1 price because you explicitly told it to (z.price, where z = b1).

Dameon · Sep 27th, 2005, 8:03 PM

Don't think of public/private on an object to object basis as much as a class to class basis.

DaWei · Sep 27th, 2005, 8:10 PM

Ahhhhh, I DID misunderstand the question.... The method gets passed an invisible pointer to the actual object; that's how it KNOWS which collection of members to use. Or, to put it another way, that's how it KNOWS which object it belongs to (and thus has access to).

Dameon · Sep 27th, 2005, 8:32 PM

I believe what he is asking is, summarized:

"If a member is private in class X, then why can instance Y access said private member on instance Z?"

DaWei · Sep 27th, 2005, 9:02 PM

Yeah, I finally got it. You said the answer. I don't believe I could phrase it better. Private members can be accessed from inside the bodies of methods of the class.

aznluvsmc · Sep 27th, 2005, 11:29 PM

So essentially what you're saying is that the member functions of a class can access the private data member of another object of that class if the object is passed into the function. They are associated only at the class level and not necessarily at the object level. Is that right?

DaWei · Sep 28th, 2005, 12:41 AM

The method is a member of the class. Each instantiation is an object. The method is sicced on the problem with a note in 'is hand about who he's working for, and a key to their house.

Sep 27th, 2005, 7:54 PM	#1
aznluvsmc Hobbyist Programmer Join Date: Aug 2005 Posts: 137 Rep Power: 4	Question about passing objects into member functions Let's assume I have the following code with the data members having relevant data. (Toggle Plain Text) class book { int price; char title[21]; public: int comparePrice(book); }; int main() { book a1, b1; //assume a constructor set the variables to something a1.comparePrice(b1); return 0; } int book::comparePrice(book z) //returns 0 or 1 { int rv; if (price < z.price) rv = 0; else rv = 1; return rv; } class book { int price; char title[21]; public: int comparePrice(book); }; int main() { book a1, b1; //assume a constructor set the variables to something a1.comparePrice(b1); return 0; } int book::comparePrice(book z) //returns 0 or 1 { int rv; if (price < z.price) rv = 0; else rv = 1; return rv; } My question is since comparePrice is a member function I know it can access the object's price directly but doesn't the member function also become associated with a particular object of the class? What I mean is, how come my comparePrice() function call can access the private member of the passed book object if the function is a member of the a1 object? I hope the question is clear. This was an example from my class but I didn't have time to ask that question .

Sep 27th, 2005, 8:00 PM	#2
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	a1.comparePrice uses the a1 price; you just told it to. It compares it against the b1 price because you explicitly told it to (z.price, where z = b1). __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Sep 27th, 2005, 8:03 PM	#3
Dameon Troll Join Date: Apr 2005 Location: Texas Posts: 732 Rep Power: 4	Don't think of public/private on an object to object basis as much as a class to class basis. __________________ MD5(sig) = bcef75433db02e9ad9bf81d6f7c5c270

Sep 27th, 2005, 8:10 PM	#4
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	Ahhhhh, I DID misunderstand the question.... The method gets passed an invisible pointer to the actual object; that's how it KNOWS which collection of members to use. Or, to put it another way, that's how it KNOWS which object it belongs to (and thus has access to). __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Sep 27th, 2005, 8:32 PM	#5
Dameon Troll Join Date: Apr 2005 Location: Texas Posts: 732 Rep Power: 4	I believe what he is asking is, summarized: "If a member is private in class X, then why can instance Y access said private member on instance Z?" __________________ MD5(sig) = bcef75433db02e9ad9bf81d6f7c5c270

Sep 27th, 2005, 9:02 PM	#6
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	Yeah, I finally got it. You said the answer. I don't believe I could phrase it better. Private members can be accessed from inside the bodies of methods of the class. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Sep 27th, 2005, 11:29 PM	#7
aznluvsmc Hobbyist Programmer Join Date: Aug 2005 Posts: 137 Rep Power: 4	So essentially what you're saying is that the member functions of a class can access the private data member of another object of that class if the object is passed into the function. They are associated only at the class level and not necessarily at the object level. Is that right?

Sep 28th, 2005, 12:41 AM	#8
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	The method is a member of the class. Each instantiation is an object. The method is sicced on the problem with a note in 'is hand about who he's working for, and a key to their house. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

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Programming Forums > Application Development > C++

Question about passing objects into member functions