friend function, can't figure error

[Tampopo]

Hello, I'm following my lecture notes example and have an error popping out stating:

/tmp/cckhUI3q.o: In function `main':
friendtry2.cpp:(.text+0x9c): undefined reference to `Product::Product(int, double)'
collect2: ld returned 1 exit status

Any idea how can i solve this? Been trying different example from the nte but still having same error. Maybe i didn't declare certain things properly...

#include <iostream>

using namespace std;

class Product
{
	friend void displayInfo(Product);
	private:
		int id;
		double price;
	public:
		Product(int = 0, double = 0.0);
};

void displayInfo(Product item)
{
	cout<<"Product id is #"<<item.id<<endl;
	cout<<"roduct price is $"<<item.price<<endl;
}

int main()
{
	Product oneItem(123, 12.95);
	displayInfo(oneItem);
}

[Tampopo]

I don't know I'm doing the right thing to demonstrate friend function usage, but I solve this by defining the constructors.. like this..

correct me if im wrong

#include <iostream>

using namespace std;

class Product
{
	friend void displayInfo(Product);
	private:
		int id;
		double price;
	public:
		Product();
		Product(int, double);
};

Product::Product()
{
	id = 0;
	price = 0.0;
}

Product::Product(int i, double p)
{
	id = i;
	price = p;
}

void displayInfo(Product item)
{
	cout<<"Product id is #"<<item.id<<endl;
	cout<<"Product price is $"<<item.price<<endl;
}

int main()
{
	Product oneItem(123, 12.95);
	displayInfo(oneItem);
}