Assignment operator in a constructor argument

**chadwickstein** · 09-30-2006

Hi all - this is my first post to these forums, and I'm a novice programmer.

I've been working through the book Teach Yourself C++ in 21 days. In one example, the following class declaration appears as a private member of a larger class (called List):

Code:

class ListCell
{
public:
   ListCell( int value, ListCell *cell = 0):val(value), next(cell) {} //why the assignment operator?
   int val;
   ListCell * next;
}

I understand that this ListCell class is used to create the cells of a linked list of integers. What confuses me is the second argument of the constructor. What does the assignment operator (=) in the second argument do? I didn't know the asignment operator had any place in argument lists.

This is probably some basic syntax issue that I missed along the way. Any help?

**Mario F.** · 09-30-2006

It's a default argument. It means that you can call that function with either two or one argument. if you call it with one argument, int value will be used and the second argument will default to 0.

**chadwickstein** · 09-30-2006

Thanks Mario!

**Cat** · 09-30-2006

It specifies a default argument.

So, for example, you can do these:

ListCell list1(10); // here cell defaults to zero (NULL)
ListCell list2(5, &list1); // here we give an explicit value for cell so the 0 is ignored.

The only caveat is that if you provide a default value for, say, argument x, you need to provide a default value for every argument after x as well.

And if you have a function with multiple arguments which have defaults, and you want to specify a particular (nondefault) value for argument y, you need to specify a value for every argument before y. That is, if you had a function that had two arguments, each which had a default, and you want to set the second argument to a non-default value, you can't skip the first argument.

**chadwickstein** · 09-30-2006

Thanks Cat

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