ofstream within a class

[mich]

Hello,
I'd like manage a ofstream type "inside" a class but some problem occur...If I define outfile out the class, all is ok; if I declare and define it inside the class, this happen. Any hints please?

static ofstream _outfile("howmany2.txt", fstream::out);

class HowMany {
	static std::ofstream outfile; 	
	string _name;
	static int _objectCount;
public:
	HowMany(const string& id = "") : _name(id) { outfile.open("howmany.txt", ios::out);  _objectCount++; }
	static void print(const string& msg = "") {
		if (msg.size() != 0)
			_outfile << msg << ": ";
		_outfile << "objectCount = "
				<< _objectCount << endl;		
		outfile << "hello";
	}

error LNK2001: unresolved external symbol "private: static class std::basic_ofstream<char,struct std::char_traits<char> > HowMany::outfile" (?outfile@HowMany@@0V?$basic_ofstream@DU?$char_traits@D@std@@@std@@A)
howmany.obj : error LNK2001: unresolved external symbol "private: static class std::basic_ofstream<char,struct std::char_traits<char> > HowMany::outfile" (?outfile@HowMany@@0V?$basic_ofstream@DU?$char_traits@D@std@@@std@@A)

[Danny]

You need to define the static member somewhere outside the class body:

class HowMany{/**/};
std::ofstream HowMany::outfile;

This rule applies to all non-const static data members declared inside a class.