Linker Error: Unresolved external symbol

[AceKnocks]

Hi all,
I have these two classes with following structure

//main.cpp

class Tree {
public:
static bool flag;
Tree() { 
flag = true;
}
};

class b {
public:
static bool isValid();
};

static isValid() {
if (Tree::flag) ... some code
}

When compiling the above code, there were no errors. However, upon linking I get the following error
error LNK2001: unresolved external symbol "public: static bool Tree::flag" (?flag@Tree@@2_ NA)
fatal error LNK1120: 1 unresolved externals

What am I doing wrong here?

[AceKnocks]

Just got to know that if we need to access a static data member from a class within a file scope, we need to define it after class declaration.

http://msdn2.microsoft .com/en-us/library/b1b5y48f(VS.80) .aspx

Resolved! :)

[weaknessforcats]

That's because the static variable is not part of any object. Therefore, when is it created?? The answer is that you need to create it yourself as a static variable.

That said, why is the variable static in the first place. On sht surface this does ot appear to be a situation where a static variable should be used.

I try to avoid static variables due to problems you get into with multi-threaded programs where more than one thread can access the sam variable. I see static anything as warning that bad code follows.

[AceKnocks]

Yep, I do agree on that but here I class B has to count instances of the class A's objects created at any given point in time.

[weaknessforcats]

Yep, I do agree on that but here I class B has to count instances of the class A's objects created at any given point in time.

Check out the Observer design pattern.

You don't need a static variable to do this.

However, if you use one, you have to create it manually.