Monostate Data Members

[pzling]

Hi

I'm doing an exercise to convert an existing Factory Pattern implemented with the Singleton Pattern into a Factory Pattern implemented with the Monostate Pattern.

The Factory Pattern has 3 data members, 1) a string, 2) a double, and 3) a map.

The problem I've run into is in trying to make all the data members static, I don't know how make a data member of type map static. Doing so results in LNK2001 errors on compile. I'm not 100% as to why but my guess is that it's due to map being a variable container?

How can I make the map data member static?

Thanks

[hamsterman]

class MyClass {
   static std::map<foo, bar> my_map;
};

std::map<foo, bar> MyClass::my_map;

This should work. I'm guessing you forgot that last line. If you didn't, you'll have to show the code.

[pzling]

thank you, I did indeed.

but where is the last line supposed to go and what does it achieve?

[pzling]

i thought i'd just post my code anyway. thanks again.

Header

class PayOffFactory2
{
public:
	PayOffFactory2(){}
	PayOffFactory2(const PayOffFactory2&){}
	
	typedef PayOff* (*CreatePayOffFunction)(double);
	
	void RegisterPayOff(string, CreatePayOffFunction);
	PayOff* CreatePayOff(string PayOffId, double Strike);

	PayOffFactory2& operator=(const PayOffFactory2&)
	{
		return *this;
	}
	~PayOffFactory2(){}
	
private:
	static map<string, CreatePayOffFunction> TheCreatorFunctions;
	static string PayOffId;
	static double Strike;
};

Source

void PayOffFactory2::RegisterPayOff(
	string PayOffId,
	CreatePayOffFunction CreatorFunction)
{	
	map<string, CreatePayOffFunction> TheCreatorFunctions; // <==?
	TheCreatorFunctions.insert(
		pair<string, CreatePayOffFunction>
			(PayOffId, CreatorFunction));
}

PayOff* PayOffFactory2::CreatePayOff(
	string PayOffId,
	double Strike)
{
	// Retrieve the requested PayOff
	map<string, CreatePayOffFunction>::const_iterator
		i = TheCreatorFunctions.find(PayOffId);

	if (i == TheCreatorFunctions.end())
	{
		cout << PayOffId<<" is an unknown payoff"<<endl;
		return NULL;
	}

	return (i->second)(Strike);
}

[cire]

The last line should go in an implementation file (cpp.)

In the class definition, the line is a declaration.

In the implementation file you supply the definition.

[pzling]

Thanks.

Relating back to the overarching problem, RegisterPayOff() is not working as when CreatePayOff() is called, the PayOffFactory object can't find the PayOffId in TheCreatorFunctions. I thought the problem might be because I don't have all the data members as static yet.

So does the line initialise (or create?) another instance of TheCreatorFunctions? My vague understanding of static data members was that they come into existence the moment they are declared (and so aren't defined).

Then have I inserted the line in right place above? Another compile error LNK1120 is reported.

[cire]

In RegisterPayOff() you create a local object TheCreatorFunctions. This object goes out of scope when the function returns.

The map should be defined at file scope (outside of any functions, just like you would define a global variable.)

[coder777]

As cire stated the static members must go to the implementation file. And you need to initialize them there.

That you don't get a link errror according to 'PayOffId' and 'Strike' are missing means that you never use this variables.

[pzling]

ahhh thank you all for your help. that makes a lot more sense now!