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Cannot access overloaded function which has no arguments

Posted on 2009-07-13
6
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Last Modified: 2013-12-14
Take a look at the attached code.  When I compile this (using Visual C++), I get an error on the p->load() line which says there is no 'load' function which takes zero arguments.  Why can I not inherit this function from Class1?  Note that in the case of 'loadx', which does not exist in Class2, I can call the inherited function.  Also, if I cast the Class2 pointer to Class1, I can call 'load'.

Why does C++ behave like this?
class Class1
{
public:
	int load(void)
	{
		return(5);
	}
 
	int loadx(void)
	{
		return(8);
	}
};
 
class Class2 : public Class1
{
public:
	int load(int arg)
	{
		return(7);
	}
};
 
 
int _tmain(int argc, _TCHAR* argv[])
{
	Class2 * p = new Class2();
 
	p->load();
	p->loadx();
 
	static_cast<Class1 *>(p)->load();
 
	return 0;
}

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Question by:JohnGaby
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6 Comments
 
LVL 33

Expert Comment

by:pgnatyuk
ID: 24846235
You overloaded the function from the basic class. The code looks correct.
If we will open the book, we will fine something wise like: "if B is a subclass of A, then it must be valid to use a B wherever you could use an A."
 Also it looks fine and will work:
Class2 two;
((Class1)two).load();
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LVL 9

Author Comment

by:JohnGaby
ID: 24846400
I realize that if you use a cast, you can call the load() (I mentioned that in my first post).  What I don't understand is WHY I need to have the cast at all.  There is no definition for a load function with no parameters in Class2, so why can I not call the load directly.  Note that if there is no overload of the function (as is the case for the loadx function), then I can call the function from Class1 without the cast.

I really don't like using casts, unless it is absolutely necessary, and I don't understand why it is necessary in this case.
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LVL 33

Accepted Solution

by:
pgnatyuk earned 250 total points
ID: 24846409
One more thing:
two.Class1::load();
also works.
I found this info for you: http://stackoverflow.com/questions/411103/function-with-same-name-but-different-signature-in-derived-class
shortly: "It is because name lookup stops if it finds a name in one of your bases. It won't look beyond in other bases. The function in B shadows the function in A." there is more info.
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LVL 33

Expert Comment

by:pgnatyuk
ID: 24846418
We posted almost in the same time. :) So my last comment explains it?
0
 
LVL 39

Expert Comment

by:itsmeandnobodyelse
ID: 24850103
>>>> two.Class1::load();

Yes, that's the way to go if you really think you need to have different signatures of load in baseclass and derived class.

I would name non-virtual functions of baseclass and derived class differently. Then a derived object or a pointer to derived can access all member functions both from baseclass and derived class and a baseclass pointer or baseclass object only those from the baseclass.

Overloading should be used only for virtual functions. Here calling the function for a derived object always will call the derived overload and if you want to call the basclass function you need to tell that explicitly by naming the class scope.

 
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LVL 9

Author Closing Comment

by:JohnGaby
ID: 31603062
It looks like it is by design then.  I guess I will have to live with it, although I don't understand the logic behind it.  If I am forced to use a cast, then later if I decided that I needed to add a load() to Class2 to overwrite the load() from Class1, it would not work, because of the casts.  I guess that I am simply going to have to change the name to be safe.
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