Trying to understand static members

[thatotherguy237]

Quick question, why does the compiler say "undefined reference" when I do something like this?

 //snippet of stack.h
class stack
{
	static double runningtotal;
	enum{max = 10};
	item items[10];
	int top;
public:
	stack();
	bool pop(item & theitem);
	bool push(item & theitem);
	bool isfull();
	bool isempty();
};
//snippet of stack.cpp
bool stack::pop(item & theitem)
{
	if (isempty())
		return false;
	else
	{
		top--;
/* ----> */	stack::runningtotal += theitem.payment;
	}
	return true;
}

[Disch]

C++ is weird.

Static members do not get "instantiated" by their declaration line. I still don't fully understand why.

To instantiate it, you need to put this in stack.cpp (just near the top somewhere, after stack.h is included):

double stack::runningtotal;

[thatotherguy237]

Thanks Disch, worked like a charm.

[LB]

Disch wrote:

Static members do not get "instantiated" by their declaration line. I still don't fully understand why.

Because the header file gets included by multiple source files, and it would cause linker errors?

[Disch]

Yeah I get that it's externally linked similarly to how you have to declare globals as externally linked. I guess I just think that's a oddity of the language.

[LB]

Thinking that linkage is weird (I do too) is entirely different from thinking what I thought you were thinking ;)

Hopefully when we finally get Modules this whole mess will clean itself up.