why does this have private member access?

This is a discussion on why does this have private member access? within the C++ Programming forums, part of the General Programming Boards category; I was wondering why, when you overload an operator, does it have private member access? I know it does, but ...

  1. #1
    Registered User major_small's Avatar
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    why does this have private member access?

    I was wondering why, when you overload an operator, does it have private member access? I know it does, but why? for example:

    Code:
    Class Date
    ...
    public:
         void operator=(Date&);
         ...
    private:
         int day,month,year;
    ...
    
    void Date::operator=(Date& x)
    {
         day=x.day;
         month=x.month;
         year=x.year;
    }
    Why can that be done? does it have to do with *this?
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  2. #2
    Compulsive Liar Robc's Avatar
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    Your overloaded operator is a member function. Member functions have access to the private members of a class, it doesn't necessarily have to be the same object as *this. So Date::operator= can see the private members of its Date argument.

  3. #3
    Toaster Zach L.'s Avatar
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    It is correct (as Robc was saying). The code written there is equivalent to:
    Code:
         this->day=x.day;
         this->month=x.month;
         this->year=x.year;
    Lack of source object implies that the source object is the "this" pointer. The class can access it's own internal data, regardless of what object of the class it is dealing with.

  4. #4
    Toaster Zach L.'s Avatar
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    Compile it.

  5. #5
    Registered User Micko's Avatar
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    Quote Originally Posted by Zach L.
    It is correct (as Robc was saying). The code written there is equivalent to:
    Code:
         this->day=x.day;
         this->month=x.month;
         this->year=x.year;
    I do agree that this is equivalent, but I think that this
    day=x.day and this->day=x.day is not the same regarding effeciency. Code day=x.day is more effecient because that doesn't imply a function call beacuse of the operator ->.
    Am I right?

  6. #6
    Compulsive Liar Robc's Avatar
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    >I think that this day=x.day and this->day=x.day is not the same regarding effeciency.
    I'm not a compiler writer, but I would wager than the two result in equivalent machine code. The former is just a syntactic convenience for us lowly programmers.

    >how is it possibe to compile?
    A simple statement from the standard about private members:
    A member of a class can be

    -- private; that is, its name can be used only by members and friends of the class in which it is declared.
    Note how it says "class" and not "object". This means that member functions which declare objects of the same type as *this are able to access the private members of those objects. Funky, huh?

  7. #7
    Registered User Micko's Avatar
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    >how is it possibe to compile?

    It is possible because whan you call any function memeber of a class this pointer is also sent like argument. Every acces to a memer variable is possible through this pointer.

    I'm not sure if it that would be the same machine code, I think this->... would be something like function call.
    Anyway I'd like to someone makes this clear

  8. #8
    Compulsive Liar Robc's Avatar
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    >I think this->... would be something like function call.
    Dereferencing a pointer isn't like a function call. And because this is done implicitly anyway, the two methods are probably identical in performance. I don't really have the inclination to go rooting through the standard to verify this, so I'll just say that even if they aren't identical, it really doesn't matter because the difference would be negligable.

    On one of my compilers (MVC++ .NET) the code:
    Code:
    class C {
    public:
      C();
    private:
      int i;
    };
    
    C::C()
    {
      i = 10;
      this->i = 10;
    }
    
    int main()
    {
      C c;
    }
    results in the following relevant piece of assembly code using the /Fa switch:
    Code:
    ; Line 10
    	ldarg.0				; _this$
    	ldc.i4.s	10		; i32 0xa
    	stind.i4	
    ; Line 11
    	ldarg.0				; _this$
    	ldc.i4.s	10		; i32 0xa
    	stind.i4

  9. #9
    Registered User major_small's Avatar
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    idk... I just though that it should't be able to access x.day because it's a member of a different object, but after reading the definition Robc gave, It makes perfect sense...
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  10. #10
    Toaster Zach L.'s Avatar
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    If you think about it a bit, it is perfectly in keeping with the idea of data hiding. As a quite common example, say you have a class that needs a meaningful (and non-trivial) copy constructor and/or assignment operator. If the object has a lot of internal data, then copying it, without being able to access the internal data of members of the same class would mean exposing that internal data through public accessor functions.

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