I want to use a non-static object variable as a default method argument

Question

Possible Duplicate:
c++ publicly inherited class member cannot be used as default argument
Nonstatic member as a default argument of a nonstatic member function

LinkedInteger accessElement(int index, LinkedInteger *startElement=&DataArray[0]){ // Starting at *startElement*, returns the element which is at the index of *startElement+index

    LinkedInteger NullElement;
    if (index<0){
        cout << "Index degeri sifirdan buyuk olmalidir" << endl;
        NullElement.value=0;
        NullElement.nextPtr=0;
        return NullElement;
    }       
    for (int i=0; i<index; i++){
        if (startElement->nextPtr == NULL){ // Last elements index is null.
            cout << " Erismeye calistiginiz eleman dizi sinirlarinin disindadir " << endl;
            NullElement.value=0;
            NullElement.nextPtr=0;
            return NullElement;}
        else {
            startElement=startElement->nextPtr;
        }
    }
    return *startElement; 
}

This is a method in implementation of Linked Lists in c++, which simply lets access to lists elements I want to give the header as a default argument (which is indeed DataArray[0]). It fails because of the error "invalid use of non-static data member".

this->&DataArray[0]

also fails because of "this may not be used in this context" What should I do?

Also there are some issues with the context of the code. Please ignore them.

You could use the singleton pattern maybe. – WebMonster Mar 20 '12 at 17:44 — WebMonster, Mar 20 '12 at 17:44

score 5 · Accepted Answer · answered Mar 20 '12 at 17:45

5

Use an overload:

LinkedInteger accessElement(int index){
  return accessElement(index, &DataArray[0]);
}

answered Mar 20 '12 at 17:45

Xeo

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score 1 · Answer 2 · edited Mar 20 '12 at 17:48

1

Try:

// 0 or NULL; your preference

LinkedInteger accessElement(int index, LinkedInteger *startElement = 0)
{
   if (startElement == 0)
       startElement = &DataArray[0];  // or just startElement = DataArray;
   // ...
}

edited Mar 20 '12 at 17:48

Kate Gregory

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answered Mar 20 '12 at 17:46

Kaz

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2

I like the overload answer better. But this does show one way you can get around the specific limitation about what can and cannot be used as an initializer for a default argument. – Kaz Mar 20 '12 at 17:47

score -1 · Answer 3 · answered Mar 20 '12 at 17:44

-1

Default values for arguments have to be statically known. You cannot use a runtime value as default argument.

The latter fails because it's not a valid syntax and makes no sense pretty much.

answered Mar 20 '12 at 17:44

q66

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5

2

It is wrong >> "*Default values for arguments have to be statically known*". – Nawaz Mar 20 '12 at 17:45
1

@Nawaz, hm it appears you're right; well, I guess it has to be a global at least, then. – q66 Mar 20 '12 at 17:49
1

it could be a function call as well. – Nawaz Mar 20 '12 at 17:50
Indeed it could; but that's pretty much because C++ allows you to initialize global variables from function calls. – q66 Mar 20 '12 at 17:51

I want to use a non-static object variable as a default method argument

3 Answers3